Deflake the session-level timeout test with trio's virtual clock #2536

Job	Run time
checks / pre-commit	36s
checks / readme-snippets	9s
checks / test (3.10, locked, ubuntu-latest)	47s
checks / test (3.10, lowest-direct, windows-latest)	1m 20s
checks / test (3.10, locked, windows-latest)	1m 21s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.11, lowest-direct, windows-latest)	1m 28s
checks / test (3.13, locked, ubuntu-latest)	44s
checks / test (3.12, lowest-direct, windows-latest)	1m 14s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.11, locked, windows-latest)	1m 15s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.11, lowest-direct, ubuntu-latest)	49s
checks / test (3.12, locked, windows-latest)	1m 27s
checks / test (3.13, lowest-direct, windows-latest)	2m 15s
checks / test (3.14, locked, ubuntu-latest)	1m 50s
checks / test (3.14, lowest-direct, windows-latest)	1m 17s
checks / test (3.14, locked, windows-latest)	2m 14s
checks / test (3.14, lowest-direct, ubuntu-latest)	45s
checks / test (3.13, locked, windows-latest)	1m 19s
checks / test (3.13, lowest-direct, ubuntu-latest)	49s
all-green	4s
	25m 19s

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