ci: add a problem matcher for GitHub Actions

ci: add a problem matcher for GitHub Actions

With this patch, test failures will be annotated with a helpful,
clickable message in GitHub Actions. For details, see
https://github.com/actions/toolkit/blob/master/docs/problem-matchers.md

Note: we need to set `TEST_SHELL_PATH` to Bash so that the problem
matcher is fed a file and line number for each test failure.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Đoàn Trần Công Danh <redacted>
Signed-off-by: Junio C Hamano <redacted>