Add integration test for root internal test extraction

This test verifies that root internal test files (package foo, not
foo_test) are correctly extracted when the repository has both:
1. Root-level internal tests (main_test.go with package main)
2. Nested packages with tests (nested/nested_test.go)

This scenario reproduces the bug that was fixed: the old extractor
would select the wrong package variant and miss root internal test
files.

The test ensures:
- main_test.go (root internal test) is extracted
- nested/nested_test.go (nested test) is extracted
- All test functions from both files are present in the database

This prevents regression of the bug fix.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

go/ql/integration-tests/root-internal-tests/src/go.mod

@@ -0,0 +1,3 @@
+module example.com/testpkg
+
+go 1.26

go/ql/integration-tests/root-internal-tests/src/main.go

@@ -0,0 +1,13 @@
+package main
+
+func PublicFunc() int {
+	return 42
+}
+
+func privateFunc() int {
+	return 24
+}
+
+func main() {
+	PublicFunc()
+}

go/ql/integration-tests/root-internal-tests/src/main_test.go

@@ -0,0 +1,16 @@
+package main
+
+import "testing"
+
+// Root internal test - tests private functions
+func TestPrivateFunc(t *testing.T) {
+	if privateFunc() != 24 {
+		t.Error("privateFunc failed")
+	}
+}
+
+func TestPublicFunc(t *testing.T) {
+	if PublicFunc() != 42 {
+		t.Error("PublicFunc failed")
+	}
+}

go/ql/integration-tests/root-internal-tests/src/nested/nested.go

@@ -0,0 +1,5 @@
+package nested
+
+func NestedFunc() string {
+	return "nested"
+}

go/ql/integration-tests/root-internal-tests/src/nested/nested_test.go

@@ -0,0 +1,9 @@
+package nested
+
+import "testing"
+
+func TestNestedFunc(t *testing.T) {
+	if NestedFunc() != "nested" {
+		t.Error("NestedFunc failed")
+	}
+}

go/ql/integration-tests/root-internal-tests/test.expected

@@ -0,0 +1,7 @@
+#select
+| src/main_test.go:0:0:0:0 | src/main_test.go |
+| src/nested/nested_test.go:0:0:0:0 | src/nested/nested_test.go |
+testFunctions
+| TestNestedFunc | src/nested/nested_test.go |
+| TestPrivateFunc | src/main_test.go |
+| TestPublicFunc | src/main_test.go |

go/ql/integration-tests/root-internal-tests/test.py

@@ -0,0 +1,5 @@
+import os
+
+def test(codeql, go):
+    # Test that root internal test files are extracted when nested packages have tests
+    codeql.database.create(source_root="src", extractor_option = ["extract_tests=true"])

go/ql/integration-tests/root-internal-tests/test.ql

@@ -0,0 +1,15 @@
+import go
+
+// Verify that root internal test files are extracted
+// when nested packages also have tests
+from File f
+where f.getBaseName().matches("%_test.go")
+select f.getRelativePath()
+
+query predicate testFunctions(string name, string file) {
+  exists(FuncDecl fn |
+    fn.getName().matches("Test%") and
+    name = fn.getName() and
+    file = fn.getFile().getRelativePath()
+  )
+}