tests: Extend ReadMe to mention merge into target branch
tests: Extend ReadMe to mention merge into target branch

Before testing any changes on a PR, the proposed changes are merged into
the target branch. This should be reflected in the `tests/README.md`.

justaugustus/tectonic-installer,kalmog/tectonic-installer,zbwright/tectonic-installer,mrwacky42/tectonic-installer,justaugustus/tectonic-installer,kyoto/tectonic-installer,justaugustus/tectonic-installer,lander2k2/tectonic-installer,kyoto/tectonic-installer,cpanato/tectonic-installer,mrwacky42/tectonic-installer,mrwacky42/tectonic-installer,squat/tectonic-installer,cpanato/tectonic-installer,metral/tectonic-installer,metral/tectonic-installer,metral/tectonic-installer,hhoover/tectonic-installer,lander2k2/tectonic-installer,zbwright/tectonic-installer,hhoover/tectonic-installer,kalmog/tectonic-installer,alexsomesan/tectonic-installer,alexsomesan/tectonic-installer,zbwright/tectonic-installer,squat/tectonic-installer,kyoto/tectonic-installer,lander2k2/tectonic-installer,derekhiggins/installer,alexsomesan/tectonic-installer,cpanato/tectonic-installer,hhoover/tectonic-installer,mrwacky42/tectonic-installer,kalmog/tectonic-installer,squat/tectonic-installer,derekhiggins/installer,justaugustus/tectonic-installer,kyoto/tectonic-installer,metral/tectonic-installer,squat/tectonic-installer,hhoover/tectonic-installer,alexsomesan/tectonic-installer,metral/tectonic-installer,cpanato/tectonic-installer,mrwacky42/tectonic-installer,lander2k2/tectonic-installer,zbwright/tectonic-installer,hhoover/tectonic-installer,alexsomesan/tectonic-installer,kyoto/tectonic-installer,justaugustus/tectonic-installer,zbwright/tectonic-installer