Avoid export of testonly target absl::test_allocator in CMake builds

Closes #1536

PiperOrigin-RevId: 602764437
Change-Id: Ia5c20a3874262a2ddb8797f608af17d7e86dd6d6
diff --git a/absl/container/CMakeLists.txt b/absl/container/CMakeLists.txt
index 449a2ca..ee9ca9c 100644
--- a/absl/container/CMakeLists.txt
+++ b/absl/container/CMakeLists.txt
@@ -213,6 +213,7 @@
   DEPS
     absl::config
     GTest::gmock
+  TESTONLY
 )
 
 absl_cc_test(