kola: tweaks around handling of external tests

[jlebon]

commit eca576068f79b8df16715f5651a0310b33b8cbe8
Date:   Mon Jul 13 16:24:49 2020 -0400

    kola: resolve directory when using `-E .`

    Otherwise the generated tests will have funny names like `ext...foobar`.

commit 0ab5a4a45898d2d34bceedba754d5dfde715e17f
Date:   Mon Jul 13 16:24:50 2020 -0400

    kola: add ext tests in current dir if no -E given

    E.g. when hacking on Ignition tests, I want to be able to just do
    `kola run --workdir /srv/fcos ext.ignition.foobar` from the Ignition git
    repo.

Found these useful when hacking on #1589 for tests for coreos/fedora-coreos-config#503.

[sohankunkerkar]

LGTM

[miabbott]

/lgtm

[openshift-ci-robot]

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This pull-request has been approved by: jlebon, miabbott, sohankunkerkar

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