[lsan][test] Enable many_tls_keys_pthread.cpp and disable swapcontext.cpp/fork_and_leak.cpp

With D98926, many_tls_keys_pthread.cpp appears to be working.

On glibc 2.30-0ubuntu2, swapcontext.cpp and Linux/fork_and_leak.cpp work fine
but they strangely fail on clang-cmake-aarch64-full
(https://lab.llvm.org/buildbot/#/builders/7/builds/2240).
Disable them for now.

Note: check-lsan was recently enabled on AArch64 in D98985. A test takes
10+ seconds. We should figure out the bottleneck.