[test] Move cctest/test-concurrent-string to unittests

... /objects/concurrent-string-unittest.

- Add AsciiToTwoByteString to convert ascii string to
two byte string.

Bug: v8:12781
Change-Id: Icc2f745846fa10ec940085eab0ef022d7c277e0a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3600552Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: 王澳 <wangao.james@bytedance.com>
Cr-Commit-Position: refs/heads/main@{#80101}

test/cctest/BUILD.gn

@@ -203,7 +203,6 @@ v8_source_set("cctest_sources") {
     "test-concurrent-js-array.cc",
     "test-concurrent-prototype.cc",
     "test-concurrent-script-context-table.cc",
-    "test-concurrent-string.cc",
     "test-concurrent-transition-array.cc",
     "test-constantpool.cc",
     "test-conversions.cc",

test/unittests/BUILD.gn

@@ -370,6 +370,7 @@ v8_source_set("unittests_sources") {
     "numbers/conversions-unittest.cc",
     "objects/array-list-unittest.cc",
     "objects/concurrent-descriptor-array-unittest.cc",
+    "objects/concurrent-string-unittest.cc",
     "objects/object-unittest.cc",
     "objects/osr-optimized-code-cache-unittest.cc",
     "objects/swiss-hash-table-helpers-unittest.cc",

test/unittests/test-utils.h

@@ -407,6 +407,13 @@ class V8_NODISCARD ManualGCScope final : private SaveFlags {
   ~ManualGCScope() = default;
 };
 
+static inline uint16_t* AsciiToTwoByteString(const char* source) {
+  size_t array_length = strlen(source) + 1;
+  uint16_t* converted = NewArray<uint16_t>(array_length);
+  for (size_t i = 0; i < array_length; i++) converted[i] = source[i];
+  return converted;
+}
+
 }  // namespace internal
 }  // namespace v8