Fix incorrect classpath leading to broken tests

The classpath on windows was pure fantasy and worked by fluke only if
java and all the code and dependencies where on the same drive.

if any code was on a different drive the classpath was mangled which
leads to tests failing.

e.g.  the classpath would end up being something like

D;\source\github\jenkins\remoting\target\test-classes;D;\source\github\jenkins\remoting\target\classes;C;\Users\jnord\.m2\repository\args4j\args4j\2.33\args4j-2.33.jar;C;\Users\jnord\.m2\repository\org\glassfish\tyrus\bundles\tyrus-standalone-client-jdk\2.1.5\tyrus-standalone-client-jdk-2.1.5.jar;C;\Users\jnord\.m2\repository\org\jenkins-ci\constant-pool-scanner\1.2\constant-pool-scanner-1.2.jar;C;\Users\jnord\.m2\repository\com\github\spotbugs\spotbugs-annotations\4.8.5\spotbugs-annotations-4.8.5.jar;C;\Users\jnord\.m2\repository\jakarta\websocket\jakarta.websocket-client-api\2.1.1\jakarta.websocket-client-api-2.1.1.jar;C;\Users\jnord\.m2\repository\net\jcip\jcip-annotations\1.0\jcip-annotations-1.0.jar;C;\Users\jnord\.m2\repository\org\kohsuke\access-modifier-annotation\1.33\access-modifier-annotation-1.33.jar;C;\Users\jnord\.m2\repository\org\jenkins-ci\annotation-indexer\1.17\annotation-indexer-1.17.jar;C;\Users\jnord\.m2\repository\com\google\guava\guava\31.0.1-jre\guava-31.0.1-jre.jar;C;\Users\jnord\.m2\repository\com\google\guava\failureaccess\1.0.1\failureaccess-1.0.1.jar;C;\Users\jnord\.m2\repository\com\google\guava\listenablefuture\9999.0-empty-to-avoid-conflict-with-guava\listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar;C;\Users\jnord\.m2\repository\com\google\code\findbugs\jsr305\3.0.2\jsr305-3.0.2.jar;C;\Users\jnord\.m2\repository\org\checkerframework\checker-qual\3.12.0\checker-qual-3.12.0.jar;C;\Users\jnord\.m2\repository\com\google\errorprone\error_prone_annotations\2.7.1\error_prone_annotations-2.7.1.jar;C;\Users\jnord\.m2\repository\com\google\j2objc\j2objc-annotations\1.3\j2objc-annotations-1.3.jar;C;\Users\jnord\.m2\repository\commons-io\commons-io\2.16.1\commons-io-2.16.1.jar;C;\Users\jnord\.m2\repository\org\junit\jupiter\junit-jupiter\5.10.2\junit-jupiter-5.10.2.jar;C;\Users\jnord\.m2\repository\org\junit\jupiter\junit-jupiter-api\5.10.2\junit-jupiter-api-5.10.2.jar;C;\Users\jnord\.m2\repository\org\opentest4j\opentest4j\1.3.0\opentest4j-1.3.0.jar;C;\Users\jnord\.m2\repository\org\junit\platform\junit-platform-commons\1.10.2\junit-platform-commons-1.10.2.jar;C;\Users\jnord\.m2\repository\org\junit\jupiter\junit-jupiter-engine\5.10.2\junit-jupiter-engine-5.10.2.jar;C;\Users\jnord\.m2\repository\org\junit\jupiter\junit-jupiter-params\5.10.2\junit-jupiter-params-5.10.2.jar;C;\Users\jnord\.m2\repository\org\apiguardian\apiguardian-api\1.1.2\apiguardian-api-1.1.2.jar;C;\Users\jnord\.m2\repository\org\junit\vintage\junit-vintage-engine\5.10.2\junit-vintage-engine-5.10.2.jar;C;\Users\jnord\.m2\repository\org\junit\platform\junit-platform-engine\1.10.2\junit-platform-engine-1.10.2.jar;C;\Users\jnord\.m2\repository\junit\junit\4.13.2\junit-4.13.2.jar;C;\Users\jnord\.m2\repository\org\hamcrest\hamcrest-core\2.2\hamcrest-core-2.2.jar;C;\Users\jnord\.m2\repository\org\bouncycastle\bcpkix-jdk18on\1.78.1\bcpkix-jdk18on-1.78.1.jar;C;\Users\jnord\.m2\repository\org\bouncycastle\bcutil-jdk18on\1.78.1\bcutil-jdk18on-1.78.1.jar;C;\Users\jnord\.m2\repository\org\bouncycastle\bcprov-jdk18on\1.78.1\bcprov-jdk18on-1.78.1.jar;C;\Users\jnord\.m2\repository\org\hamcrest\hamcrest\2.2\hamcrest-2.2.jar;C;\Users\jnord\.m2\repository\org\jenkins-ci\test-annotations\1.4\test-annotations-1.4.jar;C;\Users\jnord\.m2\repository\org\mockito\mockito-core\5.11.0\mockito-core-5.11.0.jar;C;\Users\jnord\.m2\repository\net\bytebuddy\byte-buddy\1.14.12\byte-buddy-1.14.12.jar;C;\Users\jnord\.m2\repository\net\bytebuddy\byte-buddy-agent\1.14.12\byte-buddy-agent-1.14.12.jar;C;\Users\jnord\.m2\repository\org\objenesis\objenesis\3.3\objenesis-3.3.jar;C;\Users\jnord\.m2\repository\org\ow2\asm\asm\9.7\asm-9.7.jar;

which leads to
Error: Unable to initialize main class hudson.remoting.Launcher

This currently works in CI and elsewhere by pure fluke that the paths
without drives
(\Users\jnord\.m2\repository\org\hamcrest\hamcrest\2.2\hamcrest-2.2.jar)
use the current drive - which in CI will always be C: for java and the
source code and maven

amends jenkinsci#495

(cherry picked from commit 1aa3806)

src/test/java/hudson/remoting/ForkRunner.java

@@ -62,13 +62,7 @@ public String getName() {
     }
 
     public String getClasspath() {
-        // this assumes we run in Maven
-        StringBuilder buf = new StringBuilder();
-        for (String entry : System.getProperty("java.class.path").split(":")) {
-            if (buf.length()>0) buf.append(File.pathSeparatorChar);
-            buf.append(entry);
-        }
-        return buf.toString();
+        return System.getProperty("java.class.path");
     }
 
     @Override