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/osmo-test.in b/tests/osmo-test.in
new file mode 100644
index 0000000..c2dc912
--- /dev/null
+++ b/tests/osmo-test.in
@@ -0,0 +1,2 @@
+abs_top_builddir=@abs_top_builddir@
+: ${LIBTOOL=$abs_top_builddir/libtool