verify the number of tests run by IonHashTests matches expectations

[pbcornell]

In particular, since each test is defined as a property of a suite object, if two distinct Ion test values from ion_hash_tests.ion result in the same test name, the last one wins by overwriting an earlier test (of the same name). We should verify that this is not occurring and thereby confirm that we have the test coverage we think we have.

[pbcornell]

The ion_hash_tests.ion file defines 166 identity expectations and 5 md5 expectations, and all the digesters defined in IonHashTests.ts (BinaryReader, BinaryWriter, ReaderSkip, TextReader, TextWriter) are performing 157 identity assertions and 5 md5 assertions. The 9 missing identity assertions are:

• redundant/duplicate tests:
  null.null // redundant with null
  1234.5 // duplicate test
  2017-01-01T00:00:00+00:00 // redundant with 2017-01-01T00:00:00Z
• skipped tests:
  $0
{$0:1}
$0::{}
• -0 issues:
  -0
0d-0
-0d-0

There's nothing particularly unexpected here, which is encouraging. Removing from M2, but keeping open as there's some cleanup to be done at some point (specifically $0 support and addressing the edge cases around -0).