The bazel-bin/ and bazel-genfiles/ directories are going to be merged
soon. That change was causing some test failures on Windows, and I
believe it is because the logic for locating test data files was getting
confused and mistakenly looking for them under bazel-bin/. This commit
updates that logic to look for a more specific file (descriptor.cc)
which does not appear under any Bazel-related directories.

It sounds like the more official solution is to use the Bazel runfiles
library (i.e. //tools/cpp/runfiles). However, I decided not to do that
because we currently still support multiple build systems, and if we
used a separate solution for Bazel then I suspect we would need even
more #ifdefs in the code to handle the different systems.