BSC_Tests: fix f_recv_next_si1(): skip unmatched RSL messages

Since Idb453fc894584ccf4f5f8b45a24421db958e9478, osmo-bsc does send
ip.access specific Measurement Pre-Processing Defaults.  This message
currently blocks the 'alt' statement in f_recv_next_si1(), so all
test cases calling it fail due to the guard timeout.

What's even worse, both TC_si_acc_rotate() and TC_si_acc_ramp_rotate()
dynamically configure the IUT in order to re-generate and send System
Information messages periodically.  If any of them fails prematurely,
the related configuration parameters would remain active, so the IUT
would continue sending System Information messages, causing failures
in subsequent test cases.

Let's simply ignore all unmatched messages in the 'alt' statement.

Change-Id: I1a85a046e1a8ebcd494354dddcbcc9707fdf5ee9
1 file changed