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/max-uint.ispc

@@ -1,8 +1,10 @@
 
-export float f_f(float a) {
-    unsigned int i = (unsigned int)a;
-    return max((unsigned int)10, i);
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float result[], uniform float aa[]) {
+    unsigned int i = (unsigned int)aa[programIndex];
+    result[programIndex] = max((unsigned int)100, i);
 }
 
-export float result() { return 10; }
+export void result(uniform float r[]) { r[programIndex] = 100; }