test: Report a file's fixtures one result each

[chfast]

A file collected from a directory was one test recording one outcome, so what happened to the fixtures beside the first had nowhere to go: a file holding two failing fixtures reported one line, and a fixture the loader declined disappeared into the file's verdict.

Running a test now returns one result per fixture, and a file which never gets as far as one — unparsable, or refused by the loader — returns the single result which says so. Collection and the counts are unchanged.

FAILED  statetest/tests1/SuiteA/test2_multi.json::test_case_1 - state root
FAILED  statetest/tests1/SuiteA/test2_multi.json::test_case_2 - state root

Ordered before #1699, which drops the second collection form: naming a file to see its fixtures listed only pays for itself while this is missing.

🤖 Generated with Claude Code

https://claude.ai/code/session_016UHPAGwcwXMjqhTLpT31K7

[codspeed-hq]

Merging this PR will not alter performance

✅ 129 untouched benchmarks

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_{Comparing test/results-per-fixture (56bab94) with master (13ec12d)}

[chfast]

Not worth its own PR: the collapse in #1699 does not need it, and evmone test wants per-fixture results in its own run_fixture_file, where the loader can decline one fixture at a time. Measured: this saves 334 lines from #1685 only by moving the same scaffolding upstream, and adds ~180 net.