Fix flaky tests in EndToEndAsyncTest in dbus_unittests.

Fix flaky tests in EndToEndAsyncTest in dbus_unittests.

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


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=114223

[hashimoto, 2011-12-13 05:01:01]

lgtm

[commit-bot: I haz the power, 2011-12-13 15:07:53]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/satorux@chromium.org/8907010/1

[commit-bot: I haz the power, 2011-12-13 17:08:44]

Change committed as 114223