Deflake the session-level timeout test with trio's virtual clock (#2788) #2537

Job	Run time
checks / pre-commit	35s
checks / readme-snippets	14s
checks / test (3.12, locked, windows-latest)	1m 26s
checks / test (3.12, locked, ubuntu-latest)	48s
checks / test (3.11, lowest-direct, ubuntu-latest)	47s
checks / test (3.10, lowest-direct, windows-latest)	1m 17s
checks / test (3.11, lowest-direct, windows-latest)	1m 16s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.14, locked, windows-latest)	2m 18s
checks / test (3.13, locked, windows-latest)	1m 21s
checks / test (3.11, locked, windows-latest)	1m 21s
checks / test (3.14, locked, ubuntu-latest)	1m 53s
checks / test (3.13, lowest-direct, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, windows-latest)	1m 37s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.10, locked, windows-latest)	1m 12s
checks / test (3.14, lowest-direct, ubuntu-latest)	51s
checks / test (3.14, lowest-direct, windows-latest)	1m 31s
checks / test (3.11, locked, ubuntu-latest)	48s
checks / test (3.13, lowest-direct, windows-latest)	1m 17s
all-green	3s
	24m 59s

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