Deal with unchecked warning when compiling tests

Compiling the tests (more accurately: re-compiling with -Xlint:unchecked) shows that javac is not fully happy:

    [javac] /path/to/descriptorparser/src/test/java/org/torproject/metrics/descriptorParser/parsers/ExtraInfoDescriptorParserTest.java:90: warning: [unchecked] unchecked cast
    [javac]             privateComputeBandwidthValues.invoke(ep, readBandwidthHistory,
    [javac]                                                 ^
    [javac]   required: SortedMap<Long,Double>
    [javac]   found:    Object