tests: apply travis clang PATH workaround

As seen in Travis build log the clang is not found, causing one of the
tests to fail.  This change adds a workaround from Travis issue 2607 to
util-linux.

Reference: https://travis-ci.org/karelzak/util-linux/jobs/33056101#L3630
Reference: https://github.com/travis-ci/travis-ci/issues/2607
Signed-off-by: Sami Kerola <kerolasa@iki.fi>