presubmit: Check errorf as well (#1845)

Uppercase all these test errors as well. And extend the presubmit to
check for these in the future. Also do a slightly smarter grep to only
get t.<something>. as (because dump regexp) this also grep over non test
files.

core/dnsserver/register_test.go

@@ -113,7 +113,7 @@ func TestGroupingServers(t *testing.T) {
 		}
 		for _, v := range test.expectedGroups {
 			if _, ok := groups[v]; !ok {
-				t.Errorf("test %d : expected value %v to be in the group, was not", i, v)
+				t.Errorf("Test %d : expected value %v to be in the group, was not", i, v)
 
 			}
 		}