issue-2809: fix data race while shutting down TFakeTestServer #7921

Job	Run time
check-running-allowed	2s
create-build-and-test-target-var	1s
Build and test (msan)	0s
Build and test (ubsan)	0s
Build and test / Start self-hosted runner (relwithdebinfo)	2m 31s
Build and test (asan) / Start self-hosted runner (release-asan)	2m 24s
Build and test (tsan) / Start self-hosted runner (release-tsan)	2m 24s
Build and test (tsan) / Build and test NBS (release-tsan) [id=dp7hbt64kqqbvpm84pp1 local_ip=10.10.10.29 ip=195.242.16.15] / Build and test release-tsan [id=dp7hbt64kqqbvpm84pp1 ip=195.242.16.15]	1h 51m 51s
Build and test (asan) / Build and test NBS (release-asan) [id=dp7kg08ttrrfadtdbf15 local_ip=10.10.10.15 ip=195.242.16.50] / Build and test release-asan [id=dp7kg08ttrrfadtdbf15 ip=195.242.16.50]	1h 45m 43s
Build and test / Build and test NBS (relwithdebinfo) [id=dp7jmh9pisorthq9k0ab local_ip=10.10.10.34 ip=195.242.16.139] / Build and test relwithdebinfo [id=dp7jmh9pisorthq9k0ab ip=195.242.16.139]	1h 35m 20s
Build and test / Sleep 0s if build failed	1s
Build and test / Release self-hosted runner (relwithdebinfo) [id=dp7jmh9pisorthq9k0ab local_ip=10.10.10.34 ip=195.242.16.139]	2m 0s
Build and test (asan) / Sleep 1s if build failed	2s
Build and test (asan) / Release self-hosted runner (release-asan) [id=dp7kg08ttrrfadtdbf15 local_ip=10.10.10.15 ip=195.242.16.50]	1m 58s
Build and test (tsan) / Sleep 1s if build failed	2s
Build and test (tsan) / Release self-hosted runner (release-tsan) [id=dp7hbt64kqqbvpm84pp1 local_ip=10.10.10.29 ip=195.242.16.15]	1m 58s
	5h 26m 17s

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