osmo-bts-trx: wait some longer before starting osmo-bts-trx

We get sporadic clock skews when running osmo-bts-trx, causing the process to
end and the test to fail. Try to give some seconds for the osmo-trx process to
sort itself out, in the hope to avoid the clock skews.

Related: OS#2325

Change-Id: I82d29358498e7ad9fef28d409808168926e2f876
1 file changed