Fix CMake build: add Clang -fbracket-depth, use GTest::gtest, drop un…

[ehds]

1. Add COPTS $<$<CXX_COMPILER_ID:Clang>:-fbracket-depth=512> to range_tracker_fuzz_test
   Clang's default bracket-depth limit is exceeded by the deeply-nested template instantiation in range_tracker_fuzz.cc, causing a compile error; raising the limit fixes the Clang build:

/usr/lib/llvm20/bin/clang++ -DCENTIPEDE_DISABLE_RIEGELI -DGTEST_LINKED_AS_SHARED_LIBRARY=1 -I/home/tcmalloc -I/home/tcmalloc/bld/_deps/fuzztest-src -I/home/tcmalloc/bld/_deps/re2-src -g -std=gnu++17 -fPIE -Wno-nullability-completeness -ftemplate-depth=1024 -MD -MT tcmalloc/internal/CMakeFiles/tcmalloc_internal_range_tracker_fuzz_test.dir/range_tracker_fuzz.cc.o -MF tcmalloc/internal/CMakeFiles/tcmalloc_internal_range_tracker_fuzz_test.dir/range_tracker_fuzz.cc.o.d -o tcmalloc/internal/CMakeFiles/tcmalloc_internal_range_tracker_fuzz_test.dir/range_tracker_fuzz.cc.o -c /home/tcmalloc/tcmalloc/internal/range_tracker_fuzz.cc
In file included from /home/tcmalloc/tcmalloc/internal/range_tracker_fuzz.cc:22:
In file included from /home/tcmalloc/bld/_deps/fuzztest-src/fuzztest/fuzztest.h:19:
In file included from /home/tcmalloc/bld/_deps/fuzztest-src/./fuzztest/domain.h:21:
In file included from /home/tcmalloc/bld/_deps/fuzztest-src/./fuzztest/domain_core.h:50:
/home/tcmalloc/bld/_deps/fuzztest-src/./fuzztest/internal/domains/aggregate_of_impl.h:47:62: fatal error: instantiating fold expression with 300 arguments exceeded expression nesting limit of 256
   47 |                            (Inner::has_custom_corpus_type || ...),
      |                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
/home/tcmalloc/bld/_deps/fuzztest-src/./fuzztest/internal/domains/aggregate_of_impl.h:54:11: note: in instantiation of template type alias 'AggregateOfImplCorpusType' requested here
   54 |           AggregateOfImplCorpusType<T, require_custom, Inner...>> {

2. tcmalloc/testing/CMakeLists.txt — change rseq_util's dependency from @com_google_googletest//:gtest_for_library to GTest::gtest

3. tcmalloc/testing/testutil.h — remove unused header filer #include "benchmark/benchmark.h"