test(pool): run C pool tests through the Rust implementation

Link poolTests against the multithreaded object set and Rust static archive,
so its C callbacks exercise the migrated pool rather than compiling a private
C implementation. Preserve ZSTD_MULTITHREAD for the test translation unit;
without it, its pthread test helpers become no-ops while the Rust pool runs
concurrently. Add a Rust regression for draining a small queue after reducing
the worker limit.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
- cargo test shrinking_the_limit_keeps_draining_a_small_queue -- --nocapture
- make -B -C tests poolTests && timeout 20s stdbuf -oL ./tests/poolTests

Refs: Rust pool migration 26b5e202

tests/poolTests.c

@@ -15,6 +15,7 @@
 #include "timefn.h"
 #include <stddef.h>
 #include <stdio.h>
+#include <time.h>
 
 #define ASSERT_TRUE(p)                                                       \
   do {                                                                       \