Revamp tests

[jlumpe]

Changes:

• Move outer @testset and using statements to runtests.jl(easier to add more outer test sets/include other files in future)
• Directly compare results against brute force searches
  • Reusable testing functions to do this for multiple trees and queries
• Test against larger data arrays
• Test multiple queries per metric
• Handle ambiguity in find_nearest when there is a tie for the kth-nearest data point
  • Explicit test case for this
• Test invalid arguments to constructor (empty data set, non-callable metric, no proper method for metric)
• Clean up code a bit

Currently, the hamming distance @testset fails due to #4.