run-tests: make sure to check if pygments is installed before using it
e80041832e introduced support to color the output of tests but used pygments
without checking whether it's installed or not. That breaks test-run-tests.t for
machines which don't have pygments installed. This patch conditionalize the
color test in test-run-tests.t and also add a check to make sure pygments is
installed before using that.
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Ran 16 tests in 0.000s
OK