go/packages: in overlay_test, allow test variant in second_file case

When CL 241577 lands, 'go list -e' will include unparseable files
(like a file without a package declaration) in GoFiles in addition to
InvalidGoFiles.

This makes go/packages more tolerant of empty test files. With
CL 241577, if a file=src.go query is given when an empty _test.go is
present, packages.Load will return two matching packages: the library
under test, and the internal test package.

This CL broadens overlay_test.go so the new behavior doesn't break the
test.

For golang/go#39986

Change-Id: I14d019129465e928a3f60a2230edd2e2d1d74687
Reviewed-on: https://go-review.googlesource.com/c/tools/+/317409
Trust: Jay Conrod <[email protected]>
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

go/packages/overlay_test.go

@@ -818,8 +818,9 @@ func testInvalidFilesBeforeOverlayContains(t *testing.T, exporter packagestest.E
 				if err != nil {
 					t.Fatal(err)
 				}
-				if len(initial) != 1 {
-					t.Fatalf("expected 1 packages, got %v", len(initial))
+				if len(initial) != 1 &&
+					(len(initial) != 2 || !isTestVariant(initial[0].ID, initial[1].ID)) {
+					t.Fatalf("expected 1 package (perhaps with test variant), got %v", len(initial))
 				}
 				pkg := initial[0]
 				if pkg.ID != tt.wantID {
@@ -849,6 +850,11 @@ func testInvalidFilesBeforeOverlayContains(t *testing.T, exporter packagestest.E
 	}
 }
 
+func isTestVariant(libID, testID string) bool {
+	variantID := fmt.Sprintf("%[1]s [%[1]s.test]", libID)
+	return variantID == testID
+}
+
 func TestInvalidXTestInGOPATH(t *testing.T) {
 	packagestest.TestAll(t, testInvalidXTestInGOPATH)
 }