[test] Make the meaning of some options clearer in --help.

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index 2dfbe385321007b49e3c1255bfa1446062c0ba19..ed63ba92b16e69bc8edb0fed9a81a9a7b0c6551b 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -290,10 +290,12 @@ def BuildOptions():
                     help="Don't run any testing variants",
                     default=False, dest="no_variants", action="store_true")
   result.add_option("--variants",
-                    help="Comma-separated list of testing variants: %s" % VARIANTS)
+                    help="Comma-separated list of testing variants;"
+                    " default: \"%s\"" % ','.join(VARIANTS))

[Michael Achenbach, 2016/07/29 11:50:25]

nit: Use "," for consistency - also below

   result.add_option("--exhaustive-variants",
                     default=False, action="store_true",
-                    help="Use exhaustive set of default variants.")
+                    help="Use exhaustive set of default variants:"
+                    " \"%s\"" % ','.join(EXHAUSTIVE_VARIANTS))
   result.add_option("--outdir", help="Base directory with compile output",
                     default="out")
   result.add_option("--predictable",