runtime: TestGdbBacktrace failures with output truncated after "Breakpoint 1" line

[tklauser]

--- FAIL: TestGdbBacktrace (1.15s)
    runtime-gdb_test.go:71: gdb version 7.12
    runtime-gdb_test.go:406: could not find '#0.*main\.eee' in backtrace
    runtime-gdb_test.go:407: gdb output:
        Loading Go Runtime support.
        Breakpoint 1 at 0x45cfa0: file /workdir/tmp/go-build673434893/main.go, line 17.
FAIL
FAIL	runtime	34.193s
FAIL

https://build.golang.org/log/0cc29adb14a85808a247fdd9a05589131e4b9f87
https://build.golang.org/log/becab673f943ca0f51eea735fd390345a5484fb1

Maybe related to #39021 and/or #39025?

[tklauser]

Also seen on a TryBot run: https://storage.googleapis.com/go-build-log/e2704994/linux-amd64_f55d3592.log

[bcmills]

Looks like this was a temporary regression in May 2020; none of these failures in over a year.

greplogs --dashboard -md -l -e 'FAIL: TestGdbBacktrace .*(?:\n .*)*Breakpoint 1 at .*\nFAIL'

2020-05-24T20:49:55-f65ad0d/linux-amd64-longtest
2020-05-23T06:47:06-b68fa57/linux-s390x-ibm
2020-05-21T14:31:36-c53b2bd/linux-amd64
2020-05-19T19:55:14-1361738/linux-s390x-ibm
2020-05-19T18:43:29-2dbbc86/linux-s390x-ibm
2020-05-19T04:10:43-c88f698/linux-386-longtest
2020-05-18T23:05:20-0d39dba/linux-386-longtest
2020-05-18T23:04:48-b0bc18d/linux-amd64-longtest
2020-05-18T13:11:11-afd477f/linux-s390x-ibm
2020-05-14T18:58:17-f474e9e/linux-amd64-staticlockranking
2020-05-14T16:53:34-1f325af/linux-amd64-longtest
2020-05-14T08:07:52-f073395/linux-s390x-ibm
2020-05-13T11:59:26-810c27e/linux-amd64-longtest
2020-05-12T15:01:56-a0698a6/linux-s390x-ibm