This change adds markdown output support to tapfile helper.
tap_generate_report() has been refactored to use low-level output
functions to write tests; TAP and markdown output is supported and both
are generated by default. Also, it should be straightforward to add
other output formats by implementing the respective low level print
functions.
The markdown output is now used by run-kola-tests.yaml to generate step
output and, if run from a PR, add a comment with test results to the PR.
Signed-off-by: Thilo Fromm <thilofromm@microsoft.com>