add locking around fake_now

add locking around fake_now

fake_now in the quictestlib is read/written by potentially many threads,
and as such should have a surrounding lock to prevent WAR/RAW errors as
caught by tsan:

2023-11-03T16:27:23.7184999Z ==================
2023-11-03T16:27:23.7185290Z WARNING: ThreadSanitizer: data race (pid=18754)
2023-11-03T16:27:23.7185720Z   Read of size 8 at 0x558f6f9fe970 by main thread:
2023-11-03T16:27:23.7186726Z     #0 qtest_create_quic_connection_ex <null> (quicapitest+0x14aead) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7187665Z     #1 qtest_create_quic_connection <null> (quicapitest+0x14b220) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7188567Z     #2 test_quic_write_read quicapitest.c (quicapitest+0x150ee2) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7189561Z     #3 run_tests <null> (quicapitest+0x2237ab) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7190294Z     #4 main <null> (quicapitest+0x223d2b) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7190720Z
2023-11-03T16:27:23.7190902Z   Previous write of size 8 at 0x558f6f9fe970 by thread T1:
2023-11-03T16:27:23.7191607Z     #0 qtest_create_quic_connection_ex <null> (quicapitest+0x14aecf) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7192505Z     #1 run_server_thread quictestlib.c (quicapitest+0x14b1d6) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7193361Z     #2 thread_run quictestlib.c (quicapitest+0x14cadf) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7193848Z
2023-11-03T16:27:23.7194220Z   Location is global 'fake_now.0' of size 8 at 0x558f6f9fe970 (quicapitest+0x1af4970)
2023-11-03T16:27:23.7194636Z
2023-11-03T16:27:23.7194816Z   Thread T1 (tid=18760, running) created by main thread at:
2023-11-03T16:27:23.7195465Z     #0 pthread_create <null> (quicapitest+0xca12d) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7196317Z     #1 qtest_create_quic_connection_ex <null> (quicapitest+0x14adcb) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7197214Z     #2 qtest_create_quic_connection <null> (quicapitest+0x14b220) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7198111Z     #3 test_quic_write_read quicapitest.c (quicapitest+0x150ee2) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7198940Z     #4 run_tests <null> (quicapitest+0x2237ab) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7199661Z     #5 main <null> (quicapitest+0x223d2b) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5)
2023-11-03T16:27:23.7200083Z
2023-11-03T16:27:23.7200862Z SUMMARY: ThreadSanitizer: data race (/home/runner/work/openssl/openssl/test/quicapitest+0x14aead) (BuildId: d06f7b04830b55de9c8482b398a1781472d1c7d5) in qtest_create_quic_connection_ex

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22616)