go/analysis/internal/checker: allow for Plan 9 reduced exit codes in tests

Because process exit status in Plan 9 is a string, not a number,
the current Plan 9 implementation of ExitError.ExitCode returns
only 1 for any sort of failure, but some tests in this package
expect an exit code of 3. Make a special case for Plan 9 to allow
for exit codes being only 0 or 1.

Fixes golang/go#68290

Change-Id: Ie0c106f70620307a2a0ed89aec742ecdc8daeffc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/596735
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Auto-Submit: Than McIntosh <[email protected]>

Author: millerresearch

go/analysis/internal/checker/fix_test.go

@@ -13,6 +13,7 @@ import (
 	"os/exec"
 	"path"
 	"regexp"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -81,7 +82,10 @@ func fix(t *testing.T, dir, analyzers string, wantExit int, patterns ...string)
 	if err, ok := err.(*exec.ExitError); !ok {
 		t.Fatalf("failed to execute multichecker: %v", err)
 	} else if err.ExitCode() != wantExit {
-		t.Errorf("exit code was %d, want %d", err.ExitCode(), wantExit)
+		// plan9 ExitCode() currently only returns 0 for success or 1 for failure
+		if !(runtime.GOOS == "plan9" && wantExit != exitCodeSuccess && err.ExitCode() != exitCodeSuccess) {
+			t.Errorf("exit code was %d, want %d", err.ExitCode(), wantExit)
+		}
 	}
 	return out
 }