Generate SkSL golden output files from test inputs during the build.

Golden SkSL outputs are intended to eventually replace the majority of
our unit tests, since they can automatically update themselves when we
change implementation details of the compiler.

If you change the compiler output without updating the Golden files, the
CheckGeneratedFiles housekeeper will be triggered. Set
`skia_compile_processors` or `skia_compile_sksl_tests` to true in your
GN args to regenerate them.

Almost all of the tests from SkSLFPTests.cpp and SkSLGLSLTests.cpp can
be migrated into separate unit-test .fp/.sksl files in a followup CL.

hcm@ has signed off on removing the copyright boilerplate preamble from
our unit test files.

Change-Id: I9e24a944bbac8f8efd62c92481b022a0b1ecdd0b
Bug: skia:10694
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/316336
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index dbdec90..399f583 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -135,7 +135,8 @@
       r'Copyright (\([cC]\) )?%s \w+' % years_pattern)
 
   for affected_file in input_api.AffectedSourceFiles(source_file_filter):
-    if 'third_party' in affected_file.LocalPath():
+    if ('third_party/' in affected_file.LocalPath() or
+        'tests/sksl/' in affected_file.LocalPath()):
       continue
     contents = input_api.ReadFile(affected_file, 'rb')
     if not re.search(copyright_pattern, contents):