Only FreeBSD needs extra PATH for bash

For the tests to run on FreeBSD, the bash executable needs to be
available in PATH.

The test_helper PATH was modified by
49a94717 and f70d958f
in order to be sure that bash is in PATH on FreeBSD.
(The default location for packages on FreeBSD is /usr/local/bin, and
since bash isn't provided by FreeBSD itself, that means /usr/local/bin
is the most common, if not only, location to find bash on FreeBSD.)

However, adding /usr/local/bin to PATH means that homebrew-managed
packages on macOS are now in PATH for the test suite. This is especially
important due to packages like readline. Since ruby-build prefers
homebrew readline when available, this makes the tests fail on macOS
(because the tests are written assuming no readline configuration.)

While the tests could stub brew in all cases, to ensure the readline
package is ignored by the tests, it seems more prudent to revert the
PATH to be as minimal as possible, and to only include /usr/local/bin by
precisely the OS that depends on it.