parallel_unittest: Wait for children to exit.
parallel_unittest: Wait for children to exit.

We expect child processes launched as parts of tests to exit before the
tearDown phase. They usually have exited by then, but we don't wait so
it ends up being a race condition. Wait for the children to exit.

BUG=chromium:325617, chromium:326796
TEST=Run the unit test.

Change-Id: I92d67979d99bc50d922bf1f142aa1f6a0142f145
Reviewed-on: https://chromium-review.googlesource.com/179324
Reviewed-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite