do work in onBefore, not in constructor

Don't want to do any work in dm if we're not going to actually run
a given test/gm

Change-Id: I16adf62729747f86de94bdd62228fbce31be0a85
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330942
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed