Fix #8345: forked test execution doesn't access main resources

Fixes issue where tests running in forked mode (Test / fork := true)
cannot access resources from src/main/resources, while non-forked
execution works correctly.

Root cause: Context classloader not set: tests using
Thread.currentThread().getContextClassLoader() couldn't find
resources because the context classloader wasn't set to the
URLClassLoader with the classpath

Solution: Set context classloader in ForkTestMain.runTests() to the
URLClassLoader containing the test classpath

worker/src/main/java/sbt/internal/worker1/ForkTestMain.java

@@ -325,6 +325,7 @@ public class ForkTestMain {
     }
 
     private void runTests(TestInfo info, ClassLoader classLoader) throws Exception {
+      Thread.currentThread().setContextClassLoader(classLoader);
       final ExecutorService executor = executorService(info.parallel);
       final TaskDef[] tests = info.taskDefs.toArray(new TaskDef[] {});
       final int nFrameworks = info.testRunners.size();