tests: Use GNU autotest to execute our tests and compare textual output

The output of make check is looking like this now:

Regression tests.

  1: bits                                            ok
  2: msgfile                                         ok
  3: sms                                             ok
  4: smscb                                           ok
  5: timer                                           FAILED (testsuite.at:38)
  6: ussd                                            FAILED (testsuite.at:44)
diff --git a/tests/bits/bitrev_test.ok b/tests/bits/bitrev_test.ok
new file mode 100644
index 0000000..47f402f
--- /dev/null
+++ b/tests/bits/bitrev_test.ok
@@ -0,0 +1,24 @@
+INORDER:  01 02 04 08 10 20 40 80 
+REVERSED: 80 40 20 10 08 04 02 01 
+
+INORDER:  02 04 08 10 20 40 80 
+REVERSED: 40 20 10 08 04 02 01 
+
+INORDER:  04 08 10 20 40 80 
+REVERSED: 20 10 08 04 02 01 
+
+INORDER:  08 10 20 40 80 
+REVERSED: 10 08 04 02 01 
+
+INORDER:  10 20 40 80 
+REVERSED: 08 04 02 01 
+
+INORDER:  20 40 80 
+REVERSED: 04 02 01 
+
+INORDER:  40 80 
+REVERSED: 02 01 
+
+INORDER:  80 
+REVERSED: 01 
+