add osmo_imsi_str_valid() and osmo_msisdn_str_valid()

Add GSM23003_IMSI_MIN_DIGITS definition.
Add regression test gsm23003_test.c to test the two new functions.

Will be used by OsmoHLR to validate VTY and CTRL input.

Change-Id: I1e94f5b0717b947d2a7a7d36bacdf04a75cb3522
diff --git a/tests/testsuite.at b/tests/testsuite.at
index 1954e66..483860f 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -287,3 +287,9 @@
 cat $abs_srcdir/prbs/prbs_test.ok > expout
 AT_CHECK([$abs_top_builddir/tests/prbs/prbs_test], [0], [expout], [ignore])
 AT_CLEANUP
+
+AT_SETUP([gsm23003])
+AT_KEYWORDS([gsm23003])
+cat $abs_srcdir/gsm23003/gsm23003_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/gsm23003/gsm23003_test], [0], [expout], [ignore])
+AT_CLEANUP