apps/bttester: Change default RPA timeout
apps/bttester: Change default RPA timeout

With larger timeout the GAP/PRIV/CONN/BI-01-C testcase is more stable
and passes 100% of the time.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble