mkosi: add new switch --without-tests/-T for running the build script without running...

mkosi: add new switch --without-tests/-T for running the build script without running tests

This new option simply controls the $WITH_TESTS environment variable
(which defaults to 1), that is supposed to be honoured by the build
script the same way as $WITH_DOCS is honoured. If set to 0 the build script
should avoid unit running tests during the build process.

This is useful for speeding up the build process, in particular when
combining this with incremental mode, as it brings build-times for mkosi
much closer to build-times on the host, when developing software.

Note that this new switch changes exactly nothing in the way mkosi puts
together image, it only controls the value of $WITH_TESTS, and it is
exclusively up to the build script to honour this or not.

My new favourite command is now:

        # mkosi -ifT shell

This command will build a new image, avoid running tests and quickly
give me a shell in it.