debuggerd_client_test: less racy test setup

Make sure that all the threads have started up, otherwise the main part
of the test might not be testing as stressful a situation as expected.
Note that the "race" moniker is still valid because of the debuggerd
timeout.

The test is now faster (405ms) when run under good conditions.

Test: atest 'debuggerd_test:debuggerd_client#race'
Test: Ran debuggerd_client.race 1000 times on its own.
Test: Ran the whole suite of debuggerd unit tests 1000 times.
Change-Id: I487e7654a71df9f1799f09c6f385c929ddf2f234