Change "variable.tracepoint arg casts in predicates" runtime test

The test currently checks on '@' output only and generates wait
tracepoint by calling "sleep 1 || wait $!", which on my fedora
does not add 'struct rusage' argument.

Adding simple wait4_ru.c program to testprogs that calls wait4
with 'struct rusage' argument and generates the proper argument
for the test.

Changing bpftrace invocation to run wait4_ru via -c option, so
we are sure we will get the single count.

tests/runtime/variable

@@ -37,7 +37,6 @@ TIMEOUT 5
 AFTER dd if=/dev/random bs=3 count=1
 
 NAME tracepoint arg casts in predicates
-RUN bpftrace -v -e 'tracepoint:syscalls:sys_enter_wait4 /args->ru/ { @ru[tid] = args->ru; } tracepoint:syscalls:sys_exit_wait4 /@ru[tid]/ { @++; exit(); }'
-EXPECT @
+RUN bpftrace -v -e 'tracepoint:syscalls:sys_enter_wait4 /args->ru/ { @ru[tid] = args->ru; } tracepoint:syscalls:sys_exit_wait4 /@ru[tid]/ { @++; exit(); }' -c ./testprogs/wait4_ru
+EXPECT @: 1
 TIMEOUT 5
-AFTER sleep 1 || wait $!

tests/testprogs/wait4_ru.c

@@ -0,0 +1,21 @@
+#include <sys/types.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/time.h>
+#include <sys/resource.h>
+#include <sys/wait.h>
+#include <stdlib.h>
+
+int main(void)
+{
+  struct rusage rusage;
+  pid_t pid;
+
+  pid = fork();
+  if (pid == 0) {
+    exit(0);
+  }
+
+  wait4(pid, NULL, 0, &rusage);
+  return 0;
+}