test_runner: flatten TAP output when running using --test

[MoLow]

Refs: #45833 (comment)
Fixes: #45833

this removes the reporting of a test file and reports its children's tests directly
A few notable things/edge cases:

• if a test file does not have child tests (e.g an empty file, a file that does not output TAP) - we will include it in the top-level output
• if a test file has child tests, and none of them failed - but the exit code was non-zero - we will include it in the top-level output. an example of this is a test with extraneous async activity

also diff of test_runner_output_cli.out is 10X more readable when diff is set to ignore whitespaces