Make reinstall transaction safe.

* Aborting during reinstall will now restore the originally installed
  keg.
  - Change install code to pass on CannotInstallFormulaError exception
    to caller so it can be reused in reinstall.
* Add "--force-new-install" flag to force installing a new formula.

Closes #22190.

Signed-off-by: Samuel John <github@SamuelJohn.de>
reinstall cleanup
Improve python tests for brew bots

Allow `build.with?` and similar methods to be used during the test
phase. The BuildOptions (`build`) are initialized with the
`Tab.used_options` unless explicitly overwritten on the command line.

So basically `build.with?` works in `def install` and in `test do` as
one would naively expect. (For the test, gramatically it should be
`built.with?` but who cares)

If a formula was installed `--with-python`, now the tests are also
run `--with-python`. This enables us to use the `python do ... end` in
a meaningful manner.

Using `python do ... end` blocks for the tests, because the bot.brew.sh has
system python per default and we need to set the PYTHONPATH for the test.
Potentially to different values for Python 2.x and 3.x.
mongodb: create var directories in post_install.