Add `WaitGroup.wait` and `WaitGroup#spawn` (#14837) #12136

Job	Run time
x86_64-gnu-test (1.7.3)	34m 19s
deploy_api_docs	2m 15s
x86_64-gnu-test (1.8.2)	35m 10s
x86_64-gnu-test-preview_mt	5m 40s
x86_64-gnu-test (1.9.2)	33m 27s
check_format	1m 33s
x86_64-gnu-test (1.10.1)	34m 51s
x86_64-musl-test	36m 17s
x86_64-gnu-test (1.11.2)	35m 41s
x86_64-gnu-test (1.12.2)	31m 35s
x86_64-gnu-test (1.13.1)	33m 17s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 11s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 9s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	34m 1s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	35m 6s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	34m 31s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 23s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 21s
	8h 43m 47s

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