pcu: Set up PCU TDMA clock by sending initial DATA.ind

In recent osmo-pcu commits, initial fn was changed to invalid value -1,
in order to be able to detect FN jumps. previously, the initial value
was set randomly to 0, which was wrong anyway because first FN received
from the BTS could be any other FN counted by the BTS at that time.

This makes some tests fail because they send RACH.ind + RTS.ind to
receive the Imm Assignment, and the Request reference in the Imm Assign
was calculated on the invalid unset FN "-1", hence it won't match test
expectancies.

In order to fix it, simply make sure the TDMA clock is initiated by
sending a DATA.ind to the PCU before tests start doing stuff
(f_init_raw() is blocked waiting for BTS_EV_SI13_NEGO).

Related: osmo-pcu.git Change-Id I29fb27981597edc69abb976049ba41aa840488cb
Related: OS#5020
Change-Id: I00c4dd9133ec9a236bf28fb8cb0afd0615791012
1 file changed