test: add skip_if_running_under_valgrind()

Enable it in the USDT tests. The context (from 0xB10C):

> every time the tracepoint is reached a SIGTRAP is fired.
> No matter the tracepoint contents, even with an empty one.
> Valgrind intercepts SIGTRAP and aborts.

See discussion in #32374.

test/functional/interface_usdt_mempool.py

@@ -158,6 +158,7 @@ class MempoolTracepointTest(BitcoinTestFramework):
         self.skip_if_no_bitcoind_tracepoints()
         self.skip_if_no_python_bcc()
         self.skip_if_no_bpf_permissions()
+        self.skip_if_running_under_valgrind()
 
     def added_test(self):
         """Add a transaction to the mempool and make sure the tracepoint returns