Change jingle_unittests target to a test executable.

Everything seems to link now.

Tested with the following command lines:

$ gn gen out-gn
$ ninja -C out-gn/ jingle_unittests
$ out-gn/jingle_unittests

This fixes one more TODO(GYP).

BUG=None
TEST=see above
R=sergeyu@chromium.org

Review URL: https://codereview.chromium.org/929433006

Cr-Commit-Position: refs/heads/master@{#317045}
1 file changed