Boost: enforce '--with-mpi' with '--without-single'

* Building MPI support for both single and multi-threaded flavors will
  result in duplicated rules for mpi.so.  Upstream bug report
  https://svn.boost.org/trac/boost/ticket/8841.
* Require '--with-mpi' be used together with '--without-single' until
  this is fixed.
brew reinstall: Reuse options from last time

Now, we can finally stop stuggesting to
    `brew rm <foo>` and then `brew install <foo> <with-your-preferred options>

So `brew reinstall` will honor all options that have been recorded
into the INSTALL_RECEIPT.json plus if `--build-bottle` was used.
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.