replaced prove command with faster asserts

Before:

    $ time make tmp/valid/mod.valid
    prove tools/validate_mappings.pl :: --whitelist "data/whitelist.txt" --blacklist "data/blacklist.txt" --hosts "www.mod.uk "  data/mappings/mod.csv && touch tmp/valid/mod.valid
    tools/validate_mappings.pl .. ok
    All tests successful.
    Files=1, Tests=440696, 86 wallclock secs (47.90 usr  3.50 sys + 72.25 cusr  3.05 csys = 126.70 CPU)
    Result: PASS

    real    1m27.970s
    user    2m0.613s
    sys 0m6.835s

After:

    $ time make tmp/valid/mod.valid
    tools/validate_mappings.pl --whitelist "data/whitelist.txt" --blacklist "data/blacklist.txt" --hosts "www.mod.uk "  data/mappings/mod.csv && touch tmp/valid/mod.valid
    52875 lines, 440696 tests, 0 errors.

    real    0m11.409s
    user    0m11.256s
    sys 0m0.070s