test_dsa: Avoid leaking LWLock tranches.

test_dsa: Avoid leaking LWLock tranches.

Since this is a test module, leaking a couple of LWLock tranches is
fine, but we want to discourage that pattern in third-party code.
This commit teaches the module to create only one tranche and to
store its ID in shared memory for use by other backends.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/dd36d384-55df-4fc2-825c-5bc56c950fa9%40gmail.com