We should wait for signal connection, for each signal.
Otherwise, 2nd call to OnConnected() may quit the message
loop instead of OnTestSignal().
FWIW, the flakiness was introduced in crrev.com/111423
BUG=106796,107301
TEST=confirm the test no longer fails by: while true; do out/Release/dbus_unittests --gtest_filter=EndToEndAsyncTest.TestSignal || break ; done
Review URL: http://codereview.chromium.org/8907010
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@114223 0039d316-1c4b-4281-b951-d872f2087c98