test: fix stderr checking in case of failure

Tests that does not automatically create the stderr file break when the
failure branch tries to display the content of the stderr file in the
test output.
Fix status view regression where tracked files was shown as added

The changes in 3615bdee4c3d1a0c719fc6b24f25f5745a6f2d92 introduced the
regression. Since it only occurs when `tig status` is run in a repo with
a detached head it was not caught by status/on-branch-test. To rectify
this on-branch-test now checks each individual test case as well as
running them all together.