Add Bash completion support to ReFrame

[vkarak]

That will be quite useful for selecting tests.

[shahzebsiddiqui]

Try the argcomplete package. See https://buildtest.readthedocs.io/en/devel/setup.html#setting-up-auto-complete-on-buildtest-arguments this on how to do arg-completion on bash/sh and tcsh/csh.

[vkarak]

Thanks @shahzebsiddiqui for your suggestion!