x/build: longtest failures missing error output

[bcmills]

https://build.golang.org/log/dab786adef1a18622f61641285864ac9c63fb7e3 is marked with fail on the dashboard, but the word FAIL does not appear in the output file at all.

Either the output is truncated, or the last test in the log (misc/cgo/testshared) exited with a nonzero status and no output.

CC @dmitshur @toothrot @cagedmantis

[bcmills]

2020-10-29T05:07:11-15131ca/linux-386-longtest
2020-10-27T20:04:19-de2d1c3/linux-386-longtest
2020-10-26T18:29:05-b08dfba/linux-386-longtest
2020-10-22T23:20:22-ad36f87/linux-386-longtest
2020-10-21T16:19:42-214136b/linux-386-longtest
2020-10-20T08:08:15-55b2d47/linux-386-longtest
2020-10-17T02:48:03-5faa828/linux-386-longtest
2020-10-14T20:17:49-2ec71e5/linux-386-longtest
2020-10-14T15:19:33-1671509/linux-386-longtest
2020-10-14T15:19:28-52669c4/linux-386-longtest
2020-10-13T18:26:39-6b66b59/linux-386-longtest
2020-09-28T18:46:39-f33263d/windows-amd64-2008
2020-09-26T16:04:46-f5c3eda/linux-386-longtest
2020-09-16T08:49:14-e82c9bd/linux-386-longtest
2020-09-15T02:52:12-bae9cf6/windows-amd64-2008
2020-08-26T21:17:11-32c09ae/linux-386-longtest
2020-08-19T21:49:56-6b42016/linux-386-longtest
2020-08-18T20:06:41-a401718/linux-386-longtest
2020-08-17T17:26:57-a26d687/linux-386-longtest
2020-08-14T21:04:00-016e13d/linux-386-longtest
2020-08-06T17:24:10-027d724/linux-386-longtest
2020-06-01T18:13:51-8be0de1/linux-386-longtest
2020-04-08T14:51:58-0dfb051/windows-amd64-2008
2020-01-06T19:03:05-6b1a3f7/linux-386-longtest
2019-06-02T19:28:21-ce656af/linux-amd64-longtest

[bcmills]

2021-01-08T01:58:34-cab1202/linux-386-longtest
2020-12-23T17:31:18-b116404/linux-386-longtest
2020-12-22T18:36:29-c06a354/linux-386-longtest
2020-12-22T18:16:54-0aa9b47/linux-386-longtest
2020-12-21T17:56:36-53c984d/linux-386-longtest
2020-12-18T00:16:17-740851b/linux-386-longtest
2020-12-16T16:40:57-5abda26/linux-386-longtest
2020-12-14T21:09:33-d06794d/linux-386-longtest
2020-12-10T16:38:15-56b783a/linux-386-longtest
2020-12-04T18:03:43-edf60be/linux-386-longtest
2020-11-24T07:07:06-6965b01/linux-386-longtest
2020-11-12T20:58:06-062e0e5/linux-386-longtest

[bcmills]

The failures are still occurring, but they now have one extra line of output: a signal: terminated line at the end of the test.

$ greplogs --dashboard -md -l -e (?m)##### .*\n.*signal: terminated\n\z

2021-08-27T01:08:12-c927599/linux-386-longtest
2021-08-26T17:19:04-166b691/linux-386-longtest
2021-06-02T17:34:43-e11d142/linux-386-longtest
2021-05-27T15:00:58-950fa11/linux-386-longtest
2021-04-28T18:50:41-ea65a12/linux-386-longtest