Many tests use stdout and stderr to validate whether the test emitted
the correct output. Because fuchsia-test-runner.py was sending all
logs, including test output, to stdout, tests could not validate
output properly.

This change removes the runner logs from stdout and stderr entirely
with the exception of output from the test. All runner logs are still
available in the "log" file.

Fixed: https://fxbug.dev/351356417

Applied formatting suggestions from isort and black via pylsp.

Both test-panic-abort-nocapture.rs and test-panic-abort.rs assert the
stderr output of the test. On Fuchsia, if a test fails an assertion,
this output will contain a line noting the process returned the code
-1028 (ZX_TASK_RETCODE_EXCEPTION_KILL). But the asserted stderr output
lacks this note. Presumably this is because other platforms implement
-Cpanic=abort by killing the process instead of returned a status
code.