modfile: add retract directives to TestParseLax

Old versions of modfile before CL 228039 should be able to parse and
ignore retract directives with version intervals.

Updates golang/go#24031

Change-Id: I0f35261997c0704a785ea8467dc5ed0738549966
Reviewed-on: https://go-review.googlesource.com/c/mod/+/230697
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

modfile/read_test.go

@@ -74,11 +74,24 @@ func TestParseLax(t *testing.T) {
 		)
 		exclude v1.2.3
 		replace <-!!!
+		retract v1.2.3 v1.2.4
+		retract (v1.2.3, v1.2.4]
+		retract v1.2.3 (
+			key1 value1
+			key2 value2
+		)
+		require good v1.0.0
 	`)
-	_, err := ParseLax("file", badFile, nil)
+	f, err := ParseLax("file", badFile, nil)
 	if err != nil {
 		t.Fatalf("ParseLax did not ignore irrelevant errors: %v", err)
 	}
+	if f.Module == nil || f.Module.Mod.Path != "m" {
+		t.Errorf("module directive was not parsed")
+	}
+	if len(f.Require) != 1 || f.Require[0].Mod.Path != "good" {
+		t.Errorf("require directive at end of file was not parsed")
+	}
 }
 
 // Test that when files in the testdata directory are parsed