support selecting locale to use for locale variation

Previously, reprotest picked a locale at random from a short list of somewhat obscure locales, leading to inconsistent test results, and sometimes the obscure locale would cause issues (python with non-UTF-8 locales, java with RK1048 character set).

Commit 610e6cae fixed this by always using the same locale.

But... it would be nice to have a commandline flag to specify a different locale, to be able to test more obscure locales, or when an issue is specific to a particular locale.