test(daemon): stop reading the stub engine's argv before it is written

[outofcoffee]

TestCmdDaemon_StartCarriesDeployConfig failed intermittently on CI because the daemon tests could read the stub engine's argv file while it was still empty. This removes the race.

Summary

• The stub engine writes its argv to a sibling file and renames it into place, so the file only exists complete. A plain > file redirection creates and truncates the file before printf writes, and a test that waited only for the file to exist could read it in between.
• waitForFile now waits for non-empty content, and a new waitForFileContaining waits for the exact text a test goes on to assert on. The tests that read the engine log use it too, since the daemon opens the log before its first line is written and they had the same gap.
• Regression tests: the stub's argv file is never visible empty, and the wait helper skips an empty or partial file.

Implementation details

• With the old redirection put back, the new stub test fails on essentially every run (30 of 30 invocations, usually on the first iteration), so the race is easy to hit and the test catches it.
• Waiting for the asserted text, not just for non-empty content, covers a file that is written in more than one step. The assertions after each wait are unchanged.
• Only test files change.