package_test.py: fix rez-test header command with %

[e2lka]

We had a test with a command that contains a % that was raising because of a bad use of print_header in package_test.py.

print_header("Running test commange: my command 3%")
FAIL

print_header("Running test commange",  "my command 3%")
OK

[JeanChristopheMorinRodeoFX]

@nerdvegas Another easy one. In the same line as #540. Though this current PR is not just cosmetic, it is really a bug that prevents us from running rez-test in some cases.

We targetted the develop branch and hope it's the right branch to target... Let us know if it's not.

Thanks!