Improve cunumeric.prod coverage and some test fixes

[yimoj]

• Improve prod coverage to 100%
• Rearrange some of the tests into the correct test class and changed the naming.
  • test_dtype_negative --> test_dtype_integer_precision, this is not a negative test.
  • test_dtype_complex_negative is removed, parameters are merged into test_dtype_complex
  • test_axis_tuple --> moved into positive tests
• Add more comments to some of the xfail tests to indicate the repro condition
  • These failures are mentioned in NumPy and cuNumeric behave differently in 4 cases in cunumeric.prod() #736, but they're not reproducible if LEGATE_TEST=1 or CUNUMERIC_TEST=1 is not set.
• Compare exceptions in negative tests.
• Fix the coverage for cunumeric.place, some of the lines slipped through after the last change in Improve code coverage in array_split/unpackbits/place #892