MTR followup for new misc features

This task collects a couple of MTR conveniences that make debugging runs more robust and less noisy: not aborting when a test fails to start, and quieting the per-worker port output under --parallel.

Fix abort when a test fails to start (e.g. valgrind)

When no test completes - for example valgrind fails to start - MTR reads the current test name from $path_testlog (var/log/mysqltest.log), but that file may not exist, so instead of reporting the aborted test cleanly MTR dies with an obscure error. Guard the "no tests completed" path against a missing $path_testlog.

Reproduce:

mtr --valgrind main.create_or_replace

Quiet the --parallel port-assignment output

At the default verbosity level, do not print one line per worker announcing the reserved ports; it is pure noise on a parallel run. It should appear only at a higher --verbose level.

worker[5] Using MTR_BUILD_THREAD 300, with reserved ports 16000..16019

worker[4] Using MTR_BUILD_THREAD 301, with reserved ports 16020..16039

worker[1] Using MTR_BUILD_THREAD 302, with reserved ports 16040..16059

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