Revert "only set install targets when not doing unit tests"

[olifre]

This reverts commit b5855a7.

Closes #82

As explained in the linked issue, this removes the need for downstream workarounds (not using make install or manually patching CMakeLists.txt) to allow to run unit tests in distro packaging.

[olifre]

This PR now also adds:

• Not attempting to install the bundled gtest if BUILD_UNITTESTS=YES and EXTERNAL_GTEST=NO.
• Running the GitHub actions tests both with internal and external gtest.

[djw8605]

@olifre could you merge the conflicts, if you want to change what is in #79. The github actions stuff looks good.

[olifre]

@djw8605 Done, thanks for merging #79 , I missed that before opening this PR.
I have now dropped my other changes (I think the solution by @ellert is better ), and only kept the GitHub actions stuff ;-).