Blacklisted branches in tst_licenses.pl are still tested in COIN

qtqa/tests/prebuild/license/tst_licenses.pl contains logic to blacklist some branches from being tested. Given that qtqa is shared for all branches this is needed to gradually add checks for modules, see e.g. https://codereview.qt-project.org/#/c/205967/ .

Anyhow, with recent Coin these checks fail. qtqa uses 'git branch' to determine the actual branch of a module, but Coin does not clone archives directly anymore - instead source tarballs are extracted.

The git branch command therefore fails:

fatal: Not a git repository: '/home/qt/work/qt/qtwebengine/.git'

And the blacklisted branch will be tested. This has been blocking 5.6 integrations, see https://testresults.qt.io/logs/qt/qtwebengine/d58060e0daf5f8364e9e97ae9c879bda62822978/LinuxUbuntu_14_04x86_64LinuxUbuntu_14_04x86_64GCCqtci-linux-Ubuntu-14.04-x86_64-1-d1f84bTestOnly_LicenseCheck/0ff864c4f2ee8902980b9f594c565835d72ef049/test_1527172851/log.txt.gz