Fix test failures when repeating JctExtensionTest

Author: ascopes

java-compiler-testing/src/test/java/io/github/ascopes/jct/tests/unit/junit/JctExtensionTest.java

@@ -171,6 +171,9 @@ void testSomethingAgain() {
   @Test
   void beforeEachHookInitialisesAnnotatedInstanceWorkspaceFields() {
     // Given
+    InstanceWorkspaceTestCase.someIgnoredStaticWorkspace = null;
+    InstanceWorkspaceTestCase.someInvalidStaticWorkspace = null;
+
     var instance1 = new InstanceWorkspaceTestCase();
     var instance2 = new InstanceWorkspaceTestCase();
     var instance3 = new InstanceWorkspaceTestCase();