Use .zeek file suffix in unit tests

testing/btest/scripts/base/protocols/modbus/length_mismatch.zeek

@@ -0,0 +1,14 @@
+# The parser generated by BinPAC needs to handle this pcap without crashing
+# or asserting.  Specifically, pasing Function Code 23,
+# ReadWriteMultipleRegistersRequest, has a field:
+#
+#     uint16[write_quantity] &length=write_byte_count;
+#
+# And the pcap has mismatching values for those quantities.
+# The use of &length on arrays previously caused array elements to
+# be treated as already having a bounds check in the parsing-loop, which
+# is problematic in the case where (write_quantity * 2) > write_byte_count
+# as that can cause reading from a location that exceeds the end of the
+# data buffer.
+
+# @TEST-EXEC: bro -r $TRACES/modbus/4SICS-GeekLounge-151022-min.pcap