t0061: do not fail test if '.' is part of $PATH

t0061: do not fail test if '.' is part of $PATH

t0061 creates a script with an unlikely name in the current
directory and asks the run_command() API to run it without an
explicit path, expecting that the script does *not* get run.  This
obviously would not work if the $PATH does contain such an element.

Check if the running shell picks up the script without an explicit
path to it, and skip the test when it does, as the run_command() API
should also run the script in such an (insane) environment.

Reported-by: "H.Merijn Brand" <h.m.brand@xs4all.nl>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>