Clean up SkSL test compilation in preparation for Metal support.

Now uses a GN template to avoid copy-pasting the same logic for each
type of test we want to perform, and the same file to be compiled in
more than one way at a time via an extra flag to compile_sksl_tests.py.

Change-Id: I8aadedeb140d78d58a345a2bac0da3d9c77e9a19
Bug: skia:10694
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319347
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
3 files changed