lisa.tests.base: Add (TestMetric|ResultBundleBase).pretty_format()
lisa.tests.base: Add (TestMetric|ResultBundleBase).pretty_format()

Allow nicer formatting of the test metrics and result bundles, including
multiline output.

ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa