test: resolve the temporary directory in the long-line coverage test
The test passed `mktempdir()` straight to `--code-coverage=@`, which is
resolved with `absrealpath`, while the recorded source path is not always
resolved the same way. It happened to work because `-L` records an already
resolved path, but the other tracked-path tests all resolve their inputs and
this one should too.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>