This patch makes the tests (including the test driver) run entirely
inside Vagrant, which avoids calling the very slow `vagrant` driver many
times for running the tests. On my machine, `./test` runs in 22 seconds,
down from hundreds of seconds prior to this patch.
This also has the nice side effect of matching how the Travis CI tests
were run, so there's no need for a separate `test_travis` anymore.
Because 'bin/dotbot' is a hybrid sh/Python script that finds the
appropriate Python binary and then runs, the test code should not invoke
it as an sh script when sh does not source the pyenv initialization
script.
The fix implemented in this patch is to directly run 'bin/dotbot' as a
Python script using 'python', which when called from bash, will run with
the correct Python version (because bash is set up to source the pyenv
initialization script).
