[string] Skip concurrent tests if FLAG_single_generation is on

Similar to https://crrev.com/c/v8/v8/+/2270548, we can add an early
return since we will not be creating thin strings while
single_generation is on.

Bug: v8:7790, v8:10614
Change-Id: Ib6ccc00bc79058daa163920a944ad328515f667e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2948888
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75042}

test/cctest/test-concurrent-string.cc

@@ -239,6 +239,8 @@ TEST(InspectTwoByteExternalizing) {
 // Inspect a one byte string, while the main thread externalizes it. Same as
 // InspectOneByteExternalizing, but using thin strings.
 TEST(InspectOneByteExternalizing_ThinString) {
+  // We will not create a thin string if single_generation is turned on.
+  if (FLAG_single_generation) return;
   CcTest::InitializeVM();
   Isolate* isolate = CcTest::i_isolate();
 
@@ -300,6 +302,8 @@ TEST(InspectOneByteExternalizing_ThinString) {
 // bytes string. Same as InspectOneIntoTwoByteExternalizing, but using thin
 // strings.
 TEST(InspectOneIntoTwoByteExternalizing_ThinString) {
+  // We will not create a thin string if single_generation is turned on.
+  if (FLAG_single_generation) return;
   CcTest::InitializeVM();
   Isolate* isolate = CcTest::i_isolate();
 
@@ -361,6 +365,8 @@ TEST(InspectOneIntoTwoByteExternalizing_ThinString) {
 // Inspect a two byte string, while the main thread externalizes it. Same as
 // InspectTwoByteExternalizing, but using thin strings.
 TEST(InspectTwoByteExternalizing_ThinString) {
+  // We will not create a thin string if single_generation is turned on.
+  if (FLAG_single_generation) return;
   CcTest::InitializeVM();
   Isolate* isolate = CcTest::i_isolate();