[junit-run] apply platform args to junit run java exec (pantsbuild#8421)

### Problem
When a test is using a particular jvm platform, it should also use that platform's args. See pantsbuild#8419

### Solution

This patch changes the junit run task to include those args in the jvm_options kwarg passed to execute the test suite.

### Result

The args are now passed to Java.

### Follow on work
I think this isn't the only place with this issue. I found it in jvm run as well.

src/python/pants/backend/jvm/tasks/junit_run.py

@@ -324,7 +324,7 @@ def parse_error_handler(parse_error):
               distribution=distribution,
               classpath=complete_classpath,
               main=JUnit.RUNNER_MAIN,
-              jvm_options=self.jvm_options + extra_jvm_options + list(target_jvm_options),
+              jvm_options=self.jvm_options + list(platform.args) + extra_jvm_options + list(target_jvm_options),
               args=args + batch_tests,
               workunit_factory=self.context.new_workunit,
               workunit_name='run',

tests/python/pants_test/backend/jvm/tasks/test_junit_run.py

@@ -7,6 +7,7 @@
 from textwrap import dedent
 
 from pants.backend.jvm.subsystems.junit import JUnit
+from pants.backend.jvm.subsystems.jvm_platform import JvmPlatform
 from pants.backend.jvm.targets.java_library import JavaLibrary
 from pants.backend.jvm.targets.junit_tests import JUnitTests
 from pants.backend.jvm.tasks.coverage.cobertura import Cobertura
@@ -226,6 +227,34 @@ def test_junit_runner_extra_jvm_options(self):
         }
       """))], target_name='tests/java/org/pantsbuild/foo:foo_test')
 
+  @ensure_cached(JUnitRun, expected_num_artifacts=1)
+  def test_junit_runner_platform_args(self):
+    self.make_target(
+      spec='tests/java/org/pantsbuild/foo:foo_test',
+      target_type=JUnitTests,
+      sources=['FooTest.java'],
+      test_platform='java8-extra',
+      #extra_jvm_options=['-Dexample.property=1'],
+    )
+    self.set_options_for_scope(JvmPlatform.options_scope,
+      platforms={
+        'java8-extra': {
+          'source': '8',
+          'target': '8',
+          'args': ['-Dexample.property=1'] },})
+    self._execute_junit_runner([('FooTest.java', dedent("""
+        package org.pantsbuild.foo;
+        import org.junit.Test;
+        import static org.junit.Assert.assertTrue;
+        public class FooTest {
+          @Test
+          public void testFoo() {
+            String exampleProperty = System.getProperty("example.property");
+            assertTrue(exampleProperty != null && exampleProperty.equals("1"));
+          }
+        }
+      """))], target_name='tests/java/org/pantsbuild/foo:foo_test')
+
   @ensure_cached(JUnitRun, expected_num_artifacts=1)
   def test_junit_runner_multiple_extra_jvm_options(self):
     self.make_target(