On musl systems, pthread_t is an opaque pointer.

This fails to compile on 32-bit systems because it is attempting to alias a
pointer of 32 bits to an integer of 64 bits, which is invalid.
--- waypipe-v0.9.2/test/common.c.old	2024-11-18 22:29:20.000000000 +0000
+++ waypipe-v0.9.2/test/common.c	2025-08-11 07:20:46.797918115 +0000
@@ -412,7 +412,7 @@
 	pthread_t tid = pthread_self();
 	char msg[1024];
 	int nwri = 0;
-	nwri += sprintf(msg + nwri, "%" PRIx64 " [%s:%3d] ", (uint64_t)tid,
+	nwri += sprintf(msg + nwri, "%" PRIxPTR " [%s:%3d] ", (uintptr_t)tid,
 			file, line);
 
 	va_list args;