Adds a helper method to tests/util.go for waiting for N blocks to
pass. This is useful for situations when you need to wait for
multiple blocks to pass, but don't know the current block number.
In general, this is safer than using "wait for height", since the
block height could have advanced further than expected while the
test was running.
Resolves remaining point in #1283
Dev Ojha
f2a83a07f9