[TEST] Fix PercolatorTests#testSimple2

This test requires a mapping since otherwise if there is no mapping
added the percolator query might not be parsed as a query on a numeric
field since the query might arrive on a node before the dynamic mapping
reached that node.

This commit also moves the `indexService.readAllowed()` call up before
the number of percolation queries is check to make sure we fail if reads
are not allowed - there might be a query in-flight which means we need
to check another node rather than return an empty result.