prepare SealedTypesTest for -Drun.javac=enabled (#3204)

test infra structure:
+ prefer dynamic shouldRunJavac() over RUN_JAVAC where possible
+ add option to say "-Drun.javac=optIn" to let test classes opt in
   - classes opt in via new field runJavacOptIn
+ make initialization of excuse constants unconditional
+ update path detection for Java 9+ (no longer points to "/jre")
+ detect more service versions of recent java versions

let SealedClassesTest opt in to run.javac
+ add test & excuse concerning JDK-8343306

org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/regression/AbstractModuleCompilationTest.java

@@ -128,7 +128,7 @@ protected Set<String> runConformModuleTest(String[] testFiles, String commandLin
 	protected Set<String> runConformModuleTest(String[] testFiles, String commandLine, String expectedFailureOutOutputString, String expectedFailureErrOutputString, boolean shouldFlushOutputDirectory, String output,
 			JavacTestOptions options, String javacCommandLine) {
 				runConformTest(testFiles, commandLine, expectedFailureOutOutputString, expectedFailureErrOutputString, shouldFlushOutputDirectory);
-				if (RUN_JAVAC) {
+				if (shouldRunJavac()) {
 					File outputDir = new File(output);
 					final Set<String> outFiles = new HashSet<>();
 					walkOutFiles(output, outFiles, true);
@@ -212,7 +212,7 @@ protected void runNegativeModuleTest(String[] testFiles, String commandLine, Str
 	void runNegativeModuleTest(String[] testFiles, String commandLine, String expectedFailureOutOutputString, String expectedFailureErrOutputString, boolean shouldFlushOutputDirectory, String javacErrorMatch,
 			String output, JavacTestOptions options) {
 				runNegativeTest(testFiles, commandLine, expectedFailureOutOutputString, expectedFailureErrOutputString, shouldFlushOutputDirectory);
-				if (RUN_JAVAC) {
+				if (shouldRunJavac()) {
 					String[] testFileNames = new String[testFiles.length/2];
 					for (int i = 0; i < testFileNames.length; i++) {
 						testFileNames[i] = testFiles[i*2];