testlib: provide config settings to tests and use them

testlib: provide config settings to tests and use them

Make any GIT_PATH, PERL_PATH, SHELL_PATH or DIFF settings
available to test scripts and use them by default for the
specified utilities.

Also pick up GIT_MINIMUM_VERSION from any config and actually
test it if set to make sure the configured GIT_PATH is at
least that version.

Finally support TG_TEST_INSTALLED and if set use the tg found
in $PATH rather than the bin-wrappers/tg executable.

As part of these changes excess calls to 'command -v' have
also been eliminated.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>