internal/lsp, go/packages/packagestest: support testing test files

This change allows testdata files to be *_test.go files.

Change-Id: Ic771ea7c89ff2d2aabd1af8be56f9c7286da9053
Reviewed-on: https://go-review.googlesource.com/c/161317
Reviewed-by: Ian Cottrell <iancottrell@google.com>

go/packages/packagestest/export.go

@@ -136,6 +136,7 @@ func Export(t testing.TB, exporter Exporter, modules []Module) *Exported {
 			Dir:     temp,
 			Env:     append(os.Environ(), "GOPACKAGESDRIVER=off"),
 			Overlay: make(map[string][]byte),
+			Tests:   true,
 		},
 		Modules: modules,
 		temp:    temp,

internal/lsp/lsp_test.go

@@ -36,7 +36,7 @@ func testLSP(t *testing.T, exporter packagestest.Exporter) {
 	// We hardcode the expected number of test cases to ensure that all tests
 	// are being executed. If a test is added, this number must be changed.
 	const expectedCompletionsCount = 63
-	const expectedDiagnosticsCount = 13
+	const expectedDiagnosticsCount = 14
 	const expectedFormatCount = 3
 	const expectedDefinitionsCount = 16
 	const expectedTypeDefinitionsCount = 2

internal/lsp/testdata/testy/testy.go

@@ -0,0 +1,3 @@
+package testy
+
+func a() {}

internal/lsp/testdata/testy/testy_test.go

@@ -0,0 +1,8 @@
+package testy
+
+import "testing"
+
+func TestSomething(t *testing.T) { //@item(TestSomething, "TestSomething(t *testing.T)", "", "func")
+	var x int //@diag("x", "x declared but not used")
+	a()
+}