Make sure AsyncAppenderBaseTest tests have a timeout (LBGENERAL-54).

I had an issue where I wasn't able to run the tests because the build
hung after ch.qos.logback.core.util.StatusPrinterTest. By git
bisecting I found that the issue was in the
f2d15bab commit.

Eventually after much trial and error & head-scratching I stumbed upon
a fix involving making sure that each AsyncAppenderBaseTest test
method had a timeout.