cmd/go: refer to the right package in a test

The test checks that two packages aren't non-test dependencies.
There's a copy-paste typo, however.
When net/http is unexpectedly found as a dependendency,
we instead mention the other package in the error message.

Change-Id: I3232c6252255c839e08efa048f2232c192d0fb85
Reviewed-on: https://go-review.googlesource.com/c/go/+/351372
Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/go/internal/imports/scan_test.go

@@ -33,7 +33,7 @@ func TestScan(t *testing.T) {
 		}
 		if p == "net/http" {
 			// A test import but not an import
-			t.Errorf("json reported as importing encoding/binary but does not")
+			t.Errorf("json reported as importing net/http but does not")
 		}
 	}
 	if !foundBase64 {