net/http: sync: WaitGroup.Add called from multiple synctest bubbles

[prattmic]

From post-submit tests: https://ci.chromium.org/ui/p/golang/builders/ci/gotip-linux-amd64-staticlockranking/b8704554488014429329/test-results?q=ID%3ATestNewClientServerTest%2Fsynctest%2Fh1+ (takes a long time to load because the crash is associated with a passing test case due to #75284).

=== RUN   TestNewClientServerTest/synctest/h1
=== PAUSE TestNewClientServerTest/synctest/h1
=== CONT  TestNewClientServerTest/synctest/h1
--- PASS: TestNewClientServerTest/synctest/h1 (0.00s)
fatal error: sync: WaitGroup.Add called from multiple synctest bubbles

goroutine 14 gp=0xc0004a2540 m=3 mp=0xc000081008 [running, synctest bubble 2]:
runtime.fatal({0x9f67f7, 0x39})
	/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/panic.go:1249 +0x5c fp=0xc0004a0d60 sp=0xc0004a0d30 pc=0x445cbc
sync.fatal({0x9f67f7?, 0xc0004a0db0?})
	/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/panic.go:1159 +0x18 fp=0xc0004a0d80 sp=0xc0004a0d60 pc=0x47f298
sync.(*WaitGroup).Add(0xc0004a8040, 0x1)
	/home/swarming/.swarming/w/ir/x/w/goroot/src/sync/waitgroup.go:94 +0x105 fp=0xc0004a0dc0 sp=0xc0004a0d80 pc=0x490305
net/http/httptest.(*Server).StartTLS.(*Server).wrap.func2({0xab1d78, 0xc0004d6388}, 0x0)
	/home/swarming/.swarming/w/ir/x/w/goroot/src/net/http/httptest/server.go:363 +0x173 fp=0xc0004a0e38 sp=0xc0004a0dc0 pc=0x76ff53
net/http.(*conn).setState(0xc0004a62d0, {0xab1d78, 0xc0004d6388}, 0x0, 0x1)
	/home/swarming/.swarming/w/ir/x/w/goroot/src/net/http/server.go:1882 +0x119 fp=0xc0004a0e68 sp=0xc0004a0e38 pc=0x714d79
net/http.(*Server).Serve(0xc000236000, {0xaaf458, 0xc000302780})
	/home/swarming/.swarming/w/ir/x/w/goroot/src/net/http/server.go:3495 +0x41f fp=0xc0004a0f98 sp=0xc0004a0e68 pc=0x71af5f
net/http/httptest.(*Server).goServe.func1()
	/home/swarming/.swarming/w/ir/x/w/goroot/src/net/http/httptest/server.go:341 +0x55 fp=0xc0004a0fe0 sp=0xc0004a0f98 pc=0x770ff5
runtime.goexit({})
	/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/asm_amd64.s:1693 +0x1 fp=0xc0004a0fe8 sp=0xc0004a0fe0 pc=0x487521
created by net/http/httptest.(*Server).goServe in goroutine 13
	/home/swarming/.swarming/w/ir/x/w/goroot/src/net/http/httptest/server.go:339 +0x65

cc @neild

[seankhliao]

maybe this is the same as #75134

[prattmic]

Ah, maybe. I should review the fix CL.