[test] Make test-api/ScavengeExternal*String non-threaded test.

ScavengeExternalString and ScavengeExternalOneByteString clear GC flags,
which confuses concurrent sweeping that was started from another test.

Tests that mutate flags should not be threaded.

Bug: v8:7671
Change-Id: I08656d06fe85ff45baca685ebe5982528aad774c
Reviewed-on: https://chromium-review.googlesource.com/1019102Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52690}

test/cctest/test-api.cc

@@ -786,8 +786,7 @@ THREADED_TEST(NewExternalForVeryLongString) {
   }
 }
 
-
-THREADED_TEST(ScavengeExternalString) {
+TEST(ScavengeExternalString) {
   ManualGCScope manual_gc_scope;
   i::FLAG_stress_compaction = false;
   i::FLAG_gc_global = false;
@@ -811,8 +810,7 @@ THREADED_TEST(ScavengeExternalString) {
   CHECK_EQ(1, dispose_count);
 }
 
-
-THREADED_TEST(ScavengeExternalOneByteString) {
+TEST(ScavengeExternalOneByteString) {
   ManualGCScope manual_gc_scope;
   i::FLAG_stress_compaction = false;
   i::FLAG_gc_global = false;