Before this patch, test-check-config.t fails on Solaris, because
"xargs" doesn't invoke check-config.py with all filenames at once.
"xargs" may invoke specified command multiple times with part of
arguments given from stdin: according to "xargs(1)" man page, this
dividing arguments is system-dependent.
For portability of test-check-config.t, this patch adds "xargs" like
mode to check-config.py and executes it in test-check-config.t without
"xargs".
We had them on 'test-check-code-hg.t' to avoid collision with the test checking
'check-code' itself. Now that this one have been rename, we can safely remove
this suffix for all of them. This get them in line with 'check-pyflakes.t'.