This is a heavily modified version of
https://github.com/abseil/abseil-cpp/pull/1445,
which adds some missing test libraries to the test DLL.

Unlike #1445, this change moves several global variables out of
headers that did not need to be in headers.

For instance, cord_btree_exhaustive_validation was a global
defined/declared in cord_internal, but only used in cord_rep_btree
and its test.

cordz_handle defined a queue in its header even though it wasn't needed,
which also led to ODR problems.

The Spinlock used in CordzHandle is replaced with a Mutex. This was
originally a Mutex, but Chromium asked us to change it to a Spinlock
to avoid a static initializer. After this change, the static
initializer is no longer an issue.

#1407

PiperOrigin-RevId: 531516991
Change-Id: I0e431a193698b20ba03fac6e414c26f153f330a7