Add testsuite + scenarios to test nanoBTS multiTRX

Currently only 2 nanoBTS in the 900 band are attached together as a
multiTRX setup. We thus set num_trx to 2 and set channel allocator
descending to force the BTS to use the 2nd TRX when allocating channels.

Change-Id: I12e1bcb047c4efac5693cf725739e0ce2e0532ee
diff --git a/example/scenarios/mod-bts0-chanallocdescend.conf b/example/scenarios/mod-bts0-chanallocdescend.conf
new file mode 100644
index 0000000..78cd02c
--- /dev/null
+++ b/example/scenarios/mod-bts0-chanallocdescend.conf
@@ -0,0 +1,3 @@
+modifiers:
+  bts:
+  - channel_allocator: descending