The change to make the trace tests load the data files using a relative
path only worked when running the tests from the "standard" out path.
By setting the CWD to the executable dir in test setup we are able to
run the tests from any local dir when testing.

Also cleans up some repeated code in the test init switch statement.

Bug: angleproject:3630
Change-Id: I8e9e5e6e1e9cd31c5b3c7b2f52420d0ba75cf76b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2104548Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>