RFR: JDK-8179555 make run-test should always use a fresh, clean JTwork directory

[Magnus Ihse Bursie]

When running jtreg tests, make run-test should always run with a fresh, 
clean JTwork directory in order to avoid issues with code not being 
recompiled.

Bug: https://bugs.openjdk.java.net/browse/JDK-8179555
Patch inline:
diff --git a/make/RunTests.gmk b/make/RunTests.gmk
--- a/make/RunTests.gmk
+++ b/make/RunTests.gmk
@@ -407,7 +407,10 @@
      $1_JTREG_BASIC_OPTIONS += -cpa:$$(JIB_JAR)
    endif

-  run-test-$1:
+  clean-workdir-$1:
+       $$(RM) -r $$($1_TEST_SUPPORT_DIR)
+
+  run-test-$1: clean-workdir-$1
         $$(call LogWarn)
         $$(call LogWarn, Running test '$$($1_TEST)')
         $$(call MakeDir, $$($1_TEST_RESULTS_DIR) $$($1_TEST_SUPPORT_DIR))

/Magnus