.gitignore: ignore go test artifacts

go test -artifacts writes artifacts to _artifacts. These are outputs
from a specific test and are never intended to be commited.

Change-Id: I6a6a636cd7689e5e664b190c83ccb93060f26d4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/717521
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: prattmic

.gitignore

@@ -30,6 +30,7 @@ _testmain.go
 /misc/cgo/testso/main
 /pkg/
 /src/*.*/
+/src/_artifacts/
 /src/cmd/cgo/zdefaultcc.go
 /src/cmd/dist/dist
 /src/cmd/go/internal/cfg/zdefaultcc.go