Add new test-driver script, run_tests.py.

Old run_tests.sh still lives (for now).
Changes include:
- Tests are run in parallel across all of the available CPU cores
- Option to create a statically-linked executable for each test
  (rather than using the LLVM JIT).  This is in particular useful
  for AVX, which doesn't have good JIT support yet.
- Static executables also makes it possible to test x86, not
  just x86-64, codegen.
- Fixed a number of tests in failing_tests, which were actually
  failing due to the fact that the expected function signature of
  tests had changed.

failing_tests/masked-scatter-vector.ispc

@@ -14,7 +14,7 @@ export void f_fu(uniform float ret[], uniform float aa[], uniform float b) {
     varying int3 vv = array[a];
     ++vv.y;
     array[a] = vv;
-    print("fin %\n", array[programIndex].y);
+//CO    print("fin %\n", array[programIndex].y);
     ret[programIndex] = array[programIndex].y;
 }