Link Threads::Threads in tests that use threads directly

[jnooree]

Summary

Link Threads::Threads in tests that use threads directly.

Problem

Building Abseil as a monolithic shared library (-DABSL_BUILD_MONOLITHIC_SHARED_LIBS=ON with -DBUILD_SHARED_LIBS=ON) with tests enabled fails to link on platforms where libpthread is a separate DSO (glibc < 2.34):

ld: absl_lifetime_test.dir/lifetime_test.cc.o: undefined reference to symbol 'pthread_join@@GLIBC_2.2.5'
ld: /lib64/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Root cause

In monolithic-DLL mode, absl_cc_test collapses every in-DLL absl:: dependency down to the single abseil_dll target. abseil_dll links Threads::Threads privately (CMake/AbseilDll.cmake), so -lpthread is not propagated to the test executables.

• lifetime_test links no GoogleTest target (it has its own main), so its entire link line is just libabseil_dll.so. It uses std::thread directly → unresolved pthread_join → hard link failure.
• ~46 other thread-using tests only link today because GTest::gtest's imported target happens to drag -lpthread onto their link line. That is a fragile, incidental transitive dependency — exactly what breaks under the monolithic DLL once a test stops pulling GoogleTest.

Fix

Every test that references pthread symbols now depends on Threads::Threads explicitly, matching the existing convention already used by low_level_alloc_test and thread_identity_test.

Notes

• Rather than guess from source greps, the exact set was measured. All tests were built inside quay.io/pypa/manylinux_2_28_x86_64 (glibc 2.28) container with -Wl,--as-needed -lpthread appended at the end of every test link line. The binaries that retained libpthread, minus the two that already declared Threads::Threads, are exactly the 47 targets fixed here.
• Some tests — e.g. config_test, errno_saver_test, several log_* — pull pthread via thread-local storage pthread_getspecific/pthread_setspecific rather than std::thread. They still genuinely need libpthread on old glibc, so they are included.
• No change needed for Bazel. Bazel has no monolithic-DLL mode; thread-using libraries (base, malloc_internal, synchronization) carry linkopts = ["-pthread"], and Bazel propagates library linkopts transitively through deps to test binaries.