tst_qdnslookup occasionally hangs on mac

On Mac, this autotest sometimes hangs on exit.

From a sample of 500 test logs, I was able to find two instances where the test had hung:

• http://testresults.qt-project.org/ci/QtBase_master_Integration/build_02836/macx-clang_developer-build_OSX_10.7/log.txt.gz
• http://testresults.qt-project.org/ci/QtBase_master_Integration/build_02987/macx-clang_developer-build_OSX_10.7/log.txt.gz

In both cases, the testcase completes successfully and the hang occurs while the process is exiting.