Fix spotlessFiles parameter tests

- `(`,`|` and `)` should not be escaped with `\`, otherwise they get
  interpreted as literals and no file matches
- On Windows paths use backslashes, so `\\\\` has to be used.
2 for java strings and 2 since this parameter is treated as a regex.

plugin-maven/src/test/java/com/diffplug/spotless/maven/SpecificFilesTest.java

@@ -16,17 +16,29 @@
 package com.diffplug.spotless.maven;
 
 import java.io.IOException;
+import java.nio.file.Path;
 
 import org.junit.Test;
 
 public class SpecificFilesTest extends MavenIntegrationHarness {
 	private String testFile(int number, boolean absolute) throws IOException {
 		String rel = "src/main/java/test" + number + ".java";
+		Path path;
 		if (absolute) {
-			return rootFolder() + "/" + rel;
+			path = Path.of(rootFolder().getAbsolutePath(), rel);
 		} else {
-			return rel;
+			path = Path.of(rel);
 		}
+		String result = path.toString();
+		if (!isOnWindows()) {
+			return result;
+		} else {
+			return result.replace("\\", "\\\\");
+		}
+	}
+
+	private boolean isOnWindows() {
+		return System.getProperty("os.name").startsWith("Windows");
 	}
 
 	private String testFile(int number) throws IOException {
@@ -73,6 +85,11 @@ public void multiFile() throws IOException, InterruptedException {
 
 	@Test
 	public void regexp() throws IOException, InterruptedException {
-		integration(".*/src/main/java/test\\(1\\|3\\).java", true, false, true);
+		String pattern;
+		if (isOnWindows())
+			pattern = "\".*\\\\src\\\\main\\\\java\\\\test(1|3).java\"";
+		else
+			pattern = ".*/src/main/java/test(1|3).java";
+		integration(pattern, true, false, true);
 	}
 }