Prevent double initialization in codegen tests, fix ScriptGenTest#testDependentScripts

Add an assertion to prevent duplicate initialization of CodegenTestFiles
instance in CodegenTestCase, which was happening in ScriptGenTest with multiple
invocation of loadFile()

compiler/tests/org/jetbrains/kotlin/codegen/GenerateNotNullAssertionsTest.java

@@ -55,6 +55,7 @@ public class GenerateNotNullAssertionsTest extends CodegenTestCase {
         configuration.put(JVMConfigurationKeys.DISABLE_PARAM_ASSERTIONS, disableParamAssertions);
 
         myEnvironment = KotlinCoreEnvironment.createForTests(getTestRootDisposable(), configuration, EnvironmentConfigFiles.JVM_CONFIG_FILES);
+        myFiles = null;
     }
 
     private void loadSource(@NotNull String fileName) {