Create blank SkSL test files for Ninja's benefit.

GN lists both the .cpp and the .h as generated outputs, so if they don't
exist, Ninja assumes we need to rebuild the tests every time we compile.

Change-Id: I37b8b3d9e7aef1b0cb8d5c70530c2542a6c0087a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317108
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

gn/compile_sksl_tests.py

@@ -9,9 +9,9 @@ import os
 import subprocess
 import sys
 
-def unlinkIfExists(path):
+def makeEmptyFile(path):
     try:
-        os.unlink(path)
+        open(path, 'wb').close()
     except OSError:
         pass
 
@@ -47,12 +47,12 @@ for input in inputs:
                 continue
             else:
                 # The header generated an error; this counts as an overall failure for this test.
-                # Remove the passing CPP output since it's not relevant in a failure case.
-                unlinkIfExists(target + ".cpp")
+                # Blank out the passing CPP output since it's not relevant in a failure case.
+                makeEmptyFile(target + ".cpp")
         else:
-            # The CPP generated an error. We didn't actually generate a header, but there might be
-            # one from prior runs. Let's remove it for clarity.
-            unlinkIfExists(target + ".h")
+            # The CPP generated an error. We didn't actually generate a header at all, but Ninja
+            # expects an output file to exist or it won't reach steady-state.
+            makeEmptyFile(target + ".h")
     elif ext == ".sksl" or ext == ".vert":
         compile(skslc, input, target, ".glsl")
     else: