all: update go directive to 1.18

Done with:

go get [email protected]
go mod tidy
go fix ./...

Using go1.21.3.

While here, use testing.T.Helper directly and delete go19_test.go.

For golang/go#60268.

Change-Id: I924bb6d107180a04d1f879003b169c7f792d8dd4
Reviewed-on: https://go-review.googlesource.com/c/review/+/534198
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur

git-codereview/go19_test.go

@@ -14,9 +14,6 @@ import (
 	"testing"
 )
 
-// setHelper calls t.Helper() for Go 1.9+ (see go19_test.go) and does nothing otherwise.
-var setHelper = func(t *testing.T) {}
-
 func TestPendingNone(t *testing.T) {
 	gt := newGitTest(t)
 	defer gt.done()
@@ -470,12 +467,12 @@ func testPendingReply(srv *gerritServer, id, rev, status string, unresolved int)
 }
 
 func testPending(t *testing.T, want string) {
-	setHelper(t)
+	t.Helper()
 	testPendingArgs(t, nil, want)
 }
 
 func testPendingArgs(t *testing.T, args []string, want string) {
-	setHelper(t)
+	t.Helper()
 	// fake auth information to avoid Gerrit error
 	if !auth.initialized {
 		auth.initialized = true

git-codereview/pending_test.go

@@ -1,3 +1,3 @@
 module golang.org/x/review
 
-go 1.13
+go 1.18