[test] Fix jumbo build of test-icache test.

R=leszeks@chromium.org
TEST=cctest/test-icache

No-Tree-Checks: true
Change-Id: Ied26bb04d844a7585444a8bb48e12865133a4e81
Reviewed-on: https://chromium-review.googlesource.com/c/1350990
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57832}

test/cctest/test-icache.cc

@@ -11,6 +11,7 @@
 
 namespace v8 {
 namespace internal {
+namespace test_icache {
 
 using F0 = int(int);
 
@@ -189,5 +190,6 @@ TEST(TestFlushICacheOfWritableAndExecutable) {
 
 #undef __
 
+}  // namespace test_icache
 }  // namespace internal
 }  // namespace v8