Add "if_contains" to parallel_hash_map so the value does not need to be copied to be read in a thread-safe manner

Add tests for parallel_hash_map thread-safe functions.

tests/parallel_hash_map_test.cc

@@ -0,0 +1,32 @@
+#ifndef THIS_HASH_MAP
+    #define THIS_HASH_MAP  parallel_flat_hash_map
+    #define THIS_TEST_NAME ParallelFlatHashMap
+#endif
+
+namespace phmap {
+namespace container_internal {
+namespace {
+
+TEST(THIS_TEST_NAME, ThreadSafeContains) {
+    // We can't test mutable keys, or non-copyable keys with ThisMap.
+    // Test that the nodes have the proper API.
+    ThisMap<int, int> m = { {1, 7}, {2, 9} };
+    auto val = 0;
+    EXPECT_TRUE(m.contains(1, val));
+    EXPECT_EQ(val, 7);
+
+    EXPECT_FALSE(m.contains(3, val));
+
+#if __cplusplus > 199711L
+    auto func = [&val](int& v) { val = v; };
+    EXPECT_TRUE(m.if_contains(2, func));
+    EXPECT_EQ(val, 9);
+
+    EXPECT_FALSE(m.if_contains(3, func));
+#endif
+
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace phmap