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/sms/sms_test.ok b/tests/sms/sms_test.ok
new file mode 100644
index 0000000..d0e0983
--- /dev/null
+++ b/tests/sms/sms_test.ok
@@ -0,0 +1,2 @@
+SMS testing
+OK