Fix some clang-tidy warnings

test/complexity_test.cc

diff --git a/test/complexity_test.cc b/test/complexity_test.cc
index 8cf17f4..5f930e1 100644
--- a/test/complexity_test.cc
+++ b/test/complexity_test.cc
@@ -2,6 +2,7 @@
 #include <cassert>
 #include <cmath>
 #include <cstdlib>
+#include <string>
 #include <vector>
 
 #include "benchmark/benchmark.h"

test/internal_threading_test.cc

diff --git a/test/internal_threading_test.cc b/test/internal_threading_test.cc
index c57bf44..d20931a 100644
--- a/test/internal_threading_test.cc
+++ b/test/internal_threading_test.cc
@@ -2,6 +2,7 @@
 #undef NDEBUG
 
 #include <chrono>
+#include <ratio>
 #include <thread>
 
 #include "../src/timers.h"

test/locale_impermeability_test.cc

diff --git a/test/locale_impermeability_test.cc b/test/locale_impermeability_test.cc
index e2dd6cf..ec2d4d2 100644
--- a/test/locale_impermeability_test.cc
+++ b/test/locale_impermeability_test.cc
@@ -1,7 +1,6 @@
 #undef NDEBUG
 #include <cassert>
-#include <cmath>
-#include <cstdlib>
+#include <locale>
 
 #include "benchmark/benchmark.h"
 #include "output_test.h"

test/output_test_helper.cc

diff --git a/test/output_test_helper.cc b/test/output_test_helper.cc
index 43a1bfd..a4303d2 100644
--- a/test/output_test_helper.cc
+++ b/test/output_test_helper.cc
@@ -480,7 +480,7 @@
 BENCHMARK_RESTORE_DEPRECATED_WARNING
 
 int SubstrCnt(const std::string& haystack, const std::string& pat) {
-  if (pat.length() == 0) {
+  if (pat.empty()) {
     return 0;
   }
   int count = 0;

test/templated_fixture_method_test.cc

diff --git a/test/templated_fixture_method_test.cc b/test/templated_fixture_method_test.cc
index 06fc7d8..2a7ee9c 100644
--- a/test/templated_fixture_method_test.cc
+++ b/test/templated_fixture_method_test.cc
@@ -1,6 +1,5 @@
 
 #include <cassert>
-#include <memory>
 
 #include "benchmark/benchmark.h"