selftest.run: Don't use coverage and xunit by default

Usually coverage and xunit are unused and only slows down CI. This patch
disables it by default. In order to get coverage please run
`selftest/coverageall`.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #257 from ldoktor/travis

selftest.run: Don't use coverage and xunit by default