Add specific version number for test builds
With the switch to including a date in the nightly builds (legacy/trac#25099 (moved)) I encountered issues when testing patches requiring a rebuilt of older, already built bundles just because the Firefox part needed to get rebuilt due to the date change.
Let's have something like "tbb-testbuild" as the version for testbuilds instead. This should give us back the old behavior while not impacting our nightly updating project.