tests: wrap the valgrind-requiring assertion in a function

* tests/init.cfg (require_valgrind_): New function...
* tests/misc/sort-stale-thread-mem: ...extracted from here.

tests/init.cfg

@@ -160,6 +160,12 @@ require_strace_()
   fi
 }
 
+# Skip the current test if valgrind doesn't work.
+require_valgrind_()
+{
+  valgrind --help >/dev/null || skip_ "requires valgrind"
+}
+
 require_setfacl_()
 {
   setfacl -m user::rwx . \

tests/misc/sort-stale-thread-mem

@@ -22,8 +22,8 @@
 print_ver_ sort
 
 very_expensive_
+require_valgrind_
 
-valgrind --help >/dev/null || skip_ "requires valgrind"
 grep '^#define HAVE_PTHREAD_T 1' "$CONFIG_HEADER" > /dev/null ||
   skip_ 'requires pthreads'