os: fix TestRootChtimes on illumos

TestRootChtimes currently fails on illumos [1] because the times
returned by os.Stat have only microsecond precision on that builder.
Truncate them to make the test pass again.

[1] https://build.golang.org/log/9780af24c3b3073dae1d827b2b9f9e3a48912c30

Change-Id: I8cf895d0b60c854c27cb4faf57c3b44bd40bfdd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/664915
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
TryBot-Bypass: Damien Neil <[email protected]>

Author: tklauser

src/os/root_test.go

@@ -457,6 +457,9 @@ func TestRootChtimes(t *testing.T) {
 				case "js", "plan9":
 					times.atime = times.atime.Truncate(1 * time.Second)
 					times.mtime = times.mtime.Truncate(1 * time.Second)
+				case "illumos":
+					times.atime = times.atime.Truncate(1 * time.Microsecond)
+					times.mtime = times.mtime.Truncate(1 * time.Microsecond)
 				}
 
 				err := root.Chtimes(test.open, times.atime, times.mtime)