tests\auto\compilerwarnings, tests\auto\guiapplauncher and tests\auto\bic are undocumented

None of these have any README file explaining how they're supposed to be used. I was about to remove them before realising that at least one of them is probably still used somehow:

• tests/auto/bic: https://codereview.qt-project.org/#/c/198265/
  This should have had a README file in tests/auto/bic that points to https://codereview.qt-project.org/gitweb?p=qt/qtqa.git;a=blob_plain;f=tests/postbuild/bic/README;hb=HEAD when it was first added.
• tests/auto/guiapplauncher: https://codereview.qt-project.org/#/c/106032/
  Coincidentally self-approved, no documentation!
• tests/auto/compilerwarnings: https://codereview.qt-project.org/#/c/106032/
  Added in 3081c665c43b89e9f52f4be533b31d96ec34ee36.