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/syntax/dependency_graph_tests.cpp

@@ -1,3 +1,4 @@
+#include "test/cdb_helper.h"
 #include "test/temp_dir.h"
 #include "test/test.h"
 #include "command/command.h"
@@ -194,53 +195,6 @@ void write_cdb(TempDir& tmp, CompilationDatabase& cdb, llvm::StringRef json_cont
     cdb.load(tmp.path("compile_commands.json"));
 }
 
-/// Helper: build a compile_commands.json array from entries.
-/// Uses "arguments" array form to avoid platform-specific tokenization issues
-/// (e.g. TokenizeGNUCommandLine treating backslashes as escape characters).
-struct CDBEntry {
-    llvm::StringRef dir;
-    std::string file;
-    std::vector<std::string> extra_args;
-};
-
-/// Escape backslashes and quotes for JSON string values.
-std::string json_escape(llvm::StringRef s) {
-    std::string result;
-    result.reserve(s.size());
-    for(char c: s) {
-        if(c == '\\' || c == '"') {
-            result += '\\';
-        }
-        result += c;
-    }
-    return result;
-}
-
-std::string build_cdb_json(llvm::ArrayRef<CDBEntry> entries) {
-    std::string json = "[\n";
-    for(std::size_t i = 0; i < entries.size(); ++i) {
-        auto& e = entries[i];
-        if(i > 0) {
-            json += ",\n";
-        }
-        json += R"(  {"directory": ")";
-        json += json_escape(e.dir);
-        json += R"(", "file": ")";
-        json += json_escape(e.file);
-        json += R"(", "arguments": ["clang++", "-std=c++20")";
-        for(auto& arg: e.extra_args) {
-            json += R"(, ")";
-            json += json_escape(arg);
-            json += R"(")";
-        }
-        json += R"(, ")";
-        json += json_escape(e.file);
-        json += R"("]})";
-    }
-    json += "\n]";
-    return json;
-}
-
 TEST_SUITE(ScanDependencyGraph) {
 
 TEST_CASE(EmptyCDB) {