Added tests for the existing FileContentsCache, and created a superclass
that removes the duplicated code from Torque and CPP linters

R=machenbach@chromium.org,sergiyb@chromium.org
CC=​​​​yangguo@chromium.org
NOTRY=true

Bug: v8:8482
Change-Id: Ic7a0b3d58c64f395e790d4ff668fa804c05478be
Reviewed-on: https://chromium-review.googlesource.com/c/1369949
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58321}