[test] Fix flag scope __LINE__ use

`foo ## __LINE__` just emits foo__LINE__ because of how preprocessor
expansion works.

The typical solution for this is to use a CONCAT macro, but we in fact
already have a helper for what this is trying to solve,
UNIQUE_IDENTIFIER, so just use that instead.

Change-Id: Icea3f01db458c5d557e0affd3b004f4478c6c315
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3293084
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77998}

test/common/flag-utils.h

@@ -5,6 +5,8 @@
 #ifndef V8_TEST_COMMON_FLAG_UTILS_H
 #define V8_TEST_COMMON_FLAG_UTILS_H
 
+#include "src/base/macros.h"
+
 namespace v8 {
 namespace internal {
 
@@ -24,8 +26,8 @@ class V8_NODISCARD FlagScope {
 }  // namespace internal
 }  // namespace v8
 
-#define FLAG_VALUE_SCOPE(flag, value)                     \
-  v8::internal::FlagScope<bool> __scope_##flag##__LINE__( \
+#define FLAG_VALUE_SCOPE(flag, value)                              \
+  v8::internal::FlagScope<bool> UNIQUE_IDENTIFIER(__scope_##flag)( \
       &v8::internal::FLAG_##flag, value)
 #define FLAG_SCOPE(flag) FLAG_VALUE_SCOPE(flag, true)