go/tools/bazel_testing: fix Windows problems with test framework (#2311)

* When detecting the name of the main workspace, we use
  bazel.Runfile("WORKSPACE") instead of assuming the file exists.
* BazelCmd sets "--output_user_root" if we can detect it. Bazel 2.0.0
  by default puts this in some deeply nested directory by default (not
  sure how it's derived), so we hit path length limits.

Updates #2310
1 file changed