RFR: JDK-8176196 sed from FindTests.gmk prints warnings

[Magnus Ihse Bursie]

If not all JTreg group files are present, the sed expression in 
FindTests.gmk will print a warning. This is benign but irritating.

Bug: https://bugs.openjdk.java.net/browse/JDK-8176196
Patch inline:
diff --git a/make/common/FindTests.gmk b/make/common/FindTests.gmk
--- a/make/common/FindTests.gmk
+++ b/make/common/FindTests.gmk
@@ -52,7 +52,8 @@
      $1_JTREG_GROUP_FILES := $$(addprefix $$($1_JTREG_TESTROOT)/, \
          $$($1_JTREG_GROUP_FILENAMES))
      $1_JTREG_TEST_GROUPS := $$(strip $$(shell $$(SED) -n \
-        -e 's/\([^ ]*\)\w*=.*/\1/gp' $$($1_JTREG_GROUP_FILES) | 
$$(SORT) -u))
+        -e 's/\([^ ]*\)\w*=.*/\1/gp' $$(wildcard 
$$($1_JTREG_GROUP_FILES)) \
+        | $$(SORT) -u))
    endif
  endef

/Magnus