Add ability to skip tests by adding 'skip' file to test dir,

and skip simple-timing-mp-ruby test for now (until we fix ruby atomics).

tests/SConscript

@@ -275,5 +275,6 @@ os.chdir(str(Dir('.').srcdir))
 for config in configs:
     dirs = glob.glob('*/*/ref/%s/*/%s' % (env['TARGET_ISA'], config))
     for d in dirs:
+        if not os.path.exists(os.path.join(d, 'skip')):
             test_builder(env, d)
 os.chdir(cwd)

tests/quick/40.m5threads-test-atomic/ref/sparc/linux/simple-timing-mp-ruby/skip

@@ -0,0 +1 @@
+Skipping for now due to broken atomics in ruby