os/signal: TestTerminalSignal failing on linux-mips as of CL 440220

[bcmills]

https://build.golang.org/log/b10d790868d55f5ab0c97753619052684ef73a92:

session leader error: error setting tty process group back: no such processerror running second subprocess: error setting tty process group back: no such process
--- FAIL: TestTerminalSignal (0.06s)
    signal_cgo_test.go:186: Sending ^Z...
    signal_cgo_test.go:195: error reading readiness: EOF
FAIL
FAIL	os/signal	3.224s

TestTerminalSignal was rewritten in https://go.dev/cl/440220 to make direct system calls.

#56233 shows a somewhat different failure mode on s390x.

(attn @golang/mips)

[bcmills]

Per @prattmic in #56233 (comment):

On MIPS, this failure actually occurs on the second ioctl call, the first is fine. On that buildlet, strace doesn't know how to read the argument, but it looks like it is correct:

16007 1665778506.336427 write(2, "child pid: 16014 0x910000\n", 26 <unfinished ...>
16007 1665778506.336497 <... write resumed> ) = 26
16007 1665778506.336578 ioctl(3, TIOCSPGRP, 0x910000) = -1 ESRCH (No such process)
16007 1665778506.336837 write(2, "session leader error: error setting tty process group back: no such process\n", 76) = 76
16007 1665778506.336963 kill(16014, SIGKILL) = 0
16019 1665778506.337100 +++ killed by SIGKILL +++
16018 1665778506.337130 +++ killed by SIGKILL +++
16015 1665778506.337163 +++ killed by SIGKILL +++
16017 1665778506.337188 +++ killed by SIGKILL +++
16007 1665778506.337221 waitid(P_PID, 16014,  <unfinished ...>
16016 1665778506.338067 +++ killed by SIGKILL +++
16014 1665778506.338121 +++ killed by SIGKILL +++
16007 1665778506.338145 <... waitid resumed> {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=16014, si_uid=0, si_status=SIGKILL, si_utime=0, si_stime=0}, WEXITED|WNOWAIT, NULL) = 0
16007 1665778506.338224 --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=16014, si_uid=0, si_status=SIGKILL, si_utime=0, si_stime=0} ---

0x910000 is the address of pgid 16014, so that seems OK.

PID 16014 clearly exists, given that we successfully kill it, though perhaps it isn't a valid PGID, but it looks like it should:

16014 1665778506.195828 setpgid(0, 0 <unfinished ...>
16014 1665778506.195873 <... setpgid resumed> ) = 0

man 2 setpgid: "setpgid() sets the PGID of the process specified by pid to pgid. If pid is zero, then the process ID of the calling process is used. If pgid is zero, then the PGID of the process specified by pid is made the same as its process ID."

Thus, I think this is a different bug from s390x.