reexec: add reexectest package

[thaJeztah]

This package allows using the reexec functionality to execute child processes as part of a test.

[vvoland]

Hmm does it only work against test binaries created by go test -c, or does it also work with direct go test?

[thaJeztah]

Yup, should still work; adding a quick patch to show the output;

diff --git a/reexec/reexectest/reexectest_test.go b/reexec/reexectest/reexectest_test.go
index c8dd5cd..1b57bf3 100644
--- a/reexec/reexectest/reexectest_test.go
+++ b/reexec/reexectest/reexectest_test.go
@@ -33,6 +33,8 @@ func TestRun(t *testing.T) {
                }
                if got := strings.TrimSpace(strings.TrimSuffix(string(out), "PASS\n")); got != expected {
                        t.Errorf("env-and-output output: got %q, want %q", got, expected)
+               } else {
+                       t.Logf("env-and-output output: got %q", out)
                }
        })

Then run;

go test -v -run TestRun
=== RUN   TestRun
=== RUN   TestRun/env-and-output
    reexectest_test.go:37: env-and-output output: got "child-env-and-output-ok\nPASS\n"
=== RUN   TestRun/exit-code
=== RUN   TestRun/args-passthrough
=== RUN   TestRun/context
--- PASS: TestRun (0.05s)
    --- PASS: TestRun/env-and-output (0.02s)
    --- PASS: TestRun/exit-code (0.01s)
    --- PASS: TestRun/args-passthrough (0.01s)
    --- PASS: TestRun/context (0.01s)
=== RUN   TestRunNonSubtest
--- PASS: TestRunNonSubtest (0.01s)
PASS
ok  	github.com/moby/sys/reexec/reexectest	0.401s