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-1.ispc

@@ -1,19 +1,14 @@
-static float float4(uniform float a, uniform float b, uniform float c, 
-                    uniform float d) {
-    float ret = 0;
-    for (uniform int i = 0; i < programCount; i += 4) {
-        ret = insert(ret, i + 0, a);
-        ret = insert(ret, i + 1, b);
-        ret = insert(ret, i + 2, c);
-        ret = insert(ret, i + 3, d);
-    }
-    return ret;
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float r[], uniform float a[]) {
+    unsigned int i = (unsigned int)a[programIndex];
+    r[programIndex] = max((unsigned int)2, i);
 }
 
-export float f_f(float a) {
-    unsigned int i = (unsigned int)a;
-    return max((unsigned int)2, i);
+export void result(uniform float r[]) { 
+    r[programIndex] = 1+programIndex;
+    r[0] = 2;
 }
 
-export float result() { return float4(2,2,3,4); }