Test Case for Building the Index

[kuminin]

Please build the test cases for the building the index in memory.

[CrystalChun]

The expected index when building an on-disk index for: This corpus.

[CrystalChun]

Other Test Cases:

1. The index built from corpus path should create a postings.bin file for the on-disk index at the path the corpus was specified at.
2. There should be a vocab.bin file at the path the corpus was specified at with all of the vocabulary from this corpus (21 terms) written in alphabetical order.
3. A vocabTable.bin file should also be created at the path the corpus is specified at.