os/signal: TestTerminalSignal failures on Darwin builders

[bcmills]

2020-02-19T20:37:54-b15fd6b/darwin-amd64-10_11
2019-10-31T21:47:08-ef03c44/darwin-amd64-race

--- FAIL: TestTerminalSignal (10.01s)
    signal_cgo_test.go:145: "PS1='prompt> '\r\n"
    signal_cgo_test.go:145: "bash-3.2$ PS1='prompt> '\r\n"
    signal_cgo_test.go:145: "prompt> GO_TEST_TERMINAL_SIGNALS=1 /var/folders/dx/k53rs1s93538b4x20g46cj_w00\r<GNALS=1 /var/folders/dx/k53rs1s93538b4x20g46cj_w000                         \b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b0gn/T/workdir-host-darwin\r<3rs1s93538b4x20g46cj_w0000gn/T/workdir-host-darwin-                         \b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b10_11/tmp/go-build2962210\r<gn/T/workdir-host-darwin-10_11/tmp/go-build29622109                         \b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b3/b143/signal.test -test.\r<0_11/tmp/go-build296221093/b143/signal.test -test.r                         \b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\bun=TestTerminalSignal\r\n"
    signal_cgo_test.go:145: "test program entering read\r\n"
    signal_cgo_test.go:145: "^Z\r\n"
    signal_cgo_test.go:145: "[1]+  Stopped                 GO_TEST_TERMINAL_SIGNALS=1 /var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir-host-darwin-10_11/tmp/go-build296221093/b143/signal.test -test.run=TestTerminalSignal\r\n"
    signal_cgo_test.go:145: "prompt> fg\r\n"
    signal_cgo_test.go:145: "GO_TEST_TERMINAL_SIGNALS=1 /var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir-host-darwin-10_11/tmp/go-build296221093/b143/signal.test -test.run=TestTerminalSignal\r\n"
    signal_cgo_test.go:145: "\r\n"
    signal_cgo_test.go:145: "[1]+  Stopped                 GO_TEST_TERMINAL_SIGNALS=1 /var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir-host-darwin-10_11/tmp/go-build296221093/b143/signal.test -test.run=TestTerminalSignal\r\n"
    signal_cgo_test.go:145: "prompt> \r\n"
    signal_cgo_test.go:145: "prompt> exit $?\r\n"
    signal_cgo_test.go:145: "exit\r\n"
    signal_cgo_test.go:145: "There are stopped jobs.\r\n"
    signal_cgo_test.go:128: "prompt> "
    signal_cgo_test.go:237: subprogram failed: signal: killed
FAIL
FAIL	os/signal	15.203s

CC @ianlancetaylor @cherrymui @bradfitz

[bcmills]

2020-02-22T17:58:24-dad553d/darwin-amd64-10_11

[bcmills]

The test is also flaky on darwin-amd64-10_12, but with a different failure mode (a test timeout rather than signal: killed):

2020-03-13T00:19:08-85e87f9/darwin-amd64-10_12
2019-10-25T23:25:32-8c58615/darwin-amd64-10_12

[bcmills]

2020-04-20T19:36:50-0239a5c/darwin-amd64-10_15
2020-02-22T17:58:24-dad553d/darwin-amd64-10_11
2020-02-19T20:37:54-b15fd6b/darwin-amd64-10_11
2019-10-31T21:47:08-ef03c44/darwin-amd64-race