tests: Add testcase for ffi callbacks.

tests/unix/ffi_callback.py

+import sys
+try:
+    import ffi
+except ImportError:
+    print("SKIP")
+    sys.exit()
+
+libc = ffi.open("libc.so.6")
+
+qsort = libc.func("v", "qsort", "piip")
+
+def cmp(pa, pb):
+    a = ffi.as_bytearray(pa, 1)
+    b = ffi.as_bytearray(pb, 1)
+    #print("cmp:", a, b)
+    return a[0] - b[0]
+
+cmp_c = ffi.callback("i", cmp, "pp")
+
+s = bytearray(b"foobar")
+print("org string:", s)
+qsort(s, len(s), 1, cmp_c)
+print("qsort'ed:", s)

tests/unix/ffi_callback.py.exp

+org string: bytearray(b'foobar')
+qsort'ed: bytearray(b'abfoor')