nat: Implement clearing of TP-SRR flags from TPDUs

Match IMSI and destination address against a set of entries, if it
is matching the header will be modified and no sender report will be
requested. Change the test case to request the sender report and then
verify that this bit is reset to 0.
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.ok b/openbsc/tests/bsc-nat/bsc_nat_test.ok
index 2aff594..f8eee23 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.ok
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.ok
@@ -18,4 +18,7 @@
 Testing finding of a BSC Connection
 Testing rewriting MGCP messages.
 Testing MGCP response parsing.
+Testing SMSC rewriting.
+Attempting to only rewrite the HDR
+Attempting to change nothing.
 Testing execution completed.