Preallocate test case vectors in TestRegistry

Preallocating to some reasonable and small number of tests avoids
much of the geometric-reallocation threashing at low sizes, without
taking up too much memory for tiny test binaries.

src/catch2/internal/catch_test_case_registry_impl.cpp

@@ -22,6 +22,9 @@
 namespace Catch {
 
     namespace {
+        // Picked small-ish number at random
+        static size_t kInitialTestCount = 120;
+
         static void enforceNoDuplicateTestCases(
             std::vector<TestCaseHandle> const& tests ) {
             auto testInfoCmp = []( TestCaseInfo const* lhs,
@@ -123,6 +126,15 @@ namespace Catch {
         return getRegistryHub().getTestCaseRegistry().getAllTestsSorted( config );
     }
 
+
+    TestRegistry::TestRegistry() {
+        // We pre-reserve some reasonable number of tests to avoid the
+        // initial geometric growth churning during test registration.
+        m_handles.reserve( kInitialTestCount );
+        m_viewed_test_infos.reserve( kInitialTestCount );
+        m_owned_test_infos.reserve( kInitialTestCount );
+        m_invokers.reserve( kInitialTestCount );
+    }
     TestRegistry::~TestRegistry() = default;
 
     void TestRegistry::registerTest(Detail::unique_ptr<TestCaseInfo> testInfo, Detail::unique_ptr<ITestInvoker> testInvoker) {

src/catch2/internal/catch_test_case_registry_impl.hpp

@@ -36,6 +36,7 @@ namespace Catch {
         std::vector<TestCaseHandle> const& getAllTests() const override;
         std::vector<TestCaseHandle> const& getAllTestsSorted( IConfig const& config ) const override;
 
+        TestRegistry();
         ~TestRegistry() override; // = default
 
     private: