cmd/go: list test packages even when the main package has an error

List test packages (when list is run with -e) even when the main package
has an error. This is useful to get complete data for go/packages.

Fixes golang#37971

Change-Id: If6ba0270a319ea5e003d1ed8b1ad39e479e95509
Reviewed-on: https://go-review.googlesource.com/c/go/+/224944
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/list/list.go

@@ -472,9 +472,6 @@ func runList(cmd *base.Command, args []string) {
 		c := cache.Default()
 		// Add test binaries to packages to be listed.
 		for _, p := range pkgs {
-			if p.Error != nil {
-				continue
-			}
 			if len(p.TestGoFiles)+len(p.XTestGoFiles) > 0 {
 				var pmain, ptest, pxtest *load.Package
 				var err error

src/cmd/go/testdata/script/load_test_pkg_err.txt

@@ -0,0 +1,26 @@
+# Tests issue 37971. Check that tests are still loaded even when the package has an error.
+
+go list -e -test d
+cmp stdout want_stdout
+
+go list -e -test -deps d
+stdout golang.org/fake/d
+
+-- want_stdout --
+d
+d.test
+d_test [d.test]
+-- d/d.go --
+package d
+
+import "net/http"
+
+const d = http.MethodGet
+func Get() string { return d; }
+-- d/d2.go --
+-- d/d_test.go --
+package d_test
+
+import "testing"
+import "golang.org/fake/d"
+func TestD(t *testing.T) { d.Get(); }