internal/trace/v2: TestTraceStacks failures

[gopherbot]

#!watchflakes
post <- pkg == "internal/trace/v2" && test == "TestTraceStacks"

Issue created automatically to collect these failures.

Example (log):

--- FAIL: TestTraceStacks (1.69s)
    --- FAIL: TestTraceStacks/Default (1.69s)
        reader_test.go:65: unexpected error while reading the trace: expected a proc but didn't have one
        trace_test.go:569: found bad trace; dumping to test log...
        trace_test.go:570: Trace Go1.22
            EventBatch gen=1 m=18446744073709551615 time=3595135082 size=6
            Frequency freq=2208169663
            EventBatch gen=1 m=4628 time=3594260550 size=185
            ProcStart dt=25 p=6 p_seq=1
            ProcStop dt=17
...
            String id=215
            	data="runtime.(*scavengerState).park"
            String id=216
            	data="C:/workdir/go/src/runtime/mgcscavenge.go"
            String id=217
            	data="runtime.bgscavenge"
            String id=218
            	data="sync.(*WaitGroup).Wait"
            String id=219
            	data="sync.(*Mutex).Unlock"

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "internal/trace/v2" && test == "TestTraceStacks"

2023-11-10 16:18 windows-amd64-2016 go@abf84221 internal/trace/v2.TestTraceStacks (log)

--- FAIL: TestTraceStacks (1.69s)
    --- FAIL: TestTraceStacks/Default (1.69s)
        reader_test.go:65: unexpected error while reading the trace: expected a proc but didn't have one
        trace_test.go:569: found bad trace; dumping to test log...
        trace_test.go:570: Trace Go1.22
            EventBatch gen=1 m=18446744073709551615 time=3595135082 size=6
            Frequency freq=2208169663
            EventBatch gen=1 m=4628 time=3594260550 size=185
            ProcStart dt=25 p=6 p_seq=1
            ProcStop dt=17
...
            String id=215
            	data="runtime.(*scavengerState).park"
            String id=216
            	data="C:/workdir/go/src/runtime/mgcscavenge.go"
            String id=217
            	data="runtime.bgscavenge"
            String id=218
            	data="sync.(*WaitGroup).Wait"
            String id=219
            	data="sync.(*Mutex).Unlock"

— watchflakes

[mknyszek]

This issue is fixed on tip as of the resolution of #64061.