refactor(tests): use Tester fixture, normalize helpers, add index tests (#377)

## Summary

- **Use `Tester` as fixture base** for all test suites that need
compilation, replacing `TesterFixture` and removing redundant
`tester.clear()` calls (eventide zest now creates fresh instances per
TEST_CASE)
- **Remove local `Tester` variables** in `compilation_tests`,
`template_resolver_tests`, `selection_tests` — use inherited fixture
members directly
- **Normalize helper naming**: `expect_xxx` → `EXPECT_XXX`,
`go_to_definition` → `GO_TO_DEFINITION` for consistency
- **Extract shared `test/cdb_helper.h`**: deduplicate `CDBEntry`,
`json_escape`, `build_cdb_json` from `dependency_graph_tests` and
`compile_graph_integration_tests`
- **Add new test files/cases**: `project_index_tests.cpp`, expanded
`tu_index_tests`, `merged_index_tests`, `compilation_tests`

## Test plan

- [x] All existing unit tests pass
- [x] New index tests (TUIndex, MergedIndex, ProjectIndex) pass
- [x] Compilation tests (PCH, PCM, stop) pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Standardized test fixtures and helper naming, moved suites to a shared
fixture, and unified in-memory VFS and compile flows.
* Added broad new coverage: indexing, project indexing, compilation/PCH,
diagnostics, semantic features, and many targeted unit cases.
* Introduced a small compile-database helper and improved driver-style
test compilation paths.

* **Chores**
* Consolidated and reorganized test utilities and tester APIs for easier
maintenance and reuse.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/feature/signature_help_tests.cpp

@@ -8,19 +8,19 @@ namespace {
 
 namespace protocol = eventide::ipc::protocol;
 
-TEST_SUITE(SignatureHelp) {
+TEST_SUITE(SignatureHelp, Tester) {
 
-Tester tester;
 protocol::SignatureHelp help;
 
 void run(llvm::StringRef code) {
-    tester.clear();
-    tester.add_main("main.cpp", code);
-    tester.prepare();
+    add_main("main.cpp", code);
+    prepare();
 
-    tester.params.completion = {"main.cpp", tester.nameless_points()[0]};
+    auto main_path = TestVFS::path("main.cpp");
+    params.completion = {main_path, nameless_points()[0]};
+    params.add_remapped_file(main_path, sources.all_files["main.cpp"].content);
 
-    help = feature::signature_help(tester.params, {});
+    help = feature::signature_help(params, {});
 }
 
 TEST_CASE(Simple) {