[SkContext] Add unit tests for local strike cache

Add unit tests in SkStrikeCacheTest.cpp covering local strike cache options, limits, eviction, pinned strike purging, context isolation, and client text drawing on SkContext.

Change-Id: Ib32f1e22a3f6853be14f658720654d1992183bcb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1366436
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>