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/.gitignore b/.gitignore
index 4a709a7..a3bfdd2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,6 +33,16 @@
 .tarball-version
 .version
 
+#gnu autotest
+tests/package.m4
+tests/atconfig
+tests/atlocal
+tests/osmo-test
+tests/package.m4
+tests/testsuite
+tests/testsuite.dir/
+tests/testsuite.log
+
 tests/sms/sms_test
 tests/timer/timer_test
 tests/msgfile/msgfile_test