forward only -Ox to source compilation in test runner

Author: kripken

tests/runner.py

@@ -10040,7 +10040,9 @@ def setUp(self):
       Building.LLVM_OPTS = 0
       if '-O2' in self.emcc_args:
         Building.COMPILER_TEST_OPTS = [] # remove -g in -O2 tests, for more coverage
-      Building.COMPILER_TEST_OPTS += self.emcc_args # so bitcode is optimized too, this is for cpp to ll
+      for arg in self.emcc_args:
+        if arg.startswith('-O'):
+          Building.COMPILER_TEST_OPTS.append(arg) # so bitcode is optimized too, this is for cpp to ll
       return
 
     embetter = %d