Fix Unit Tests Still Using Bitstring

[medley56]

We have a number of parsing unit tests that are still using ConstBitStream as their input data. This works still but we want to eventually move everything to using PacketData instead.

This may require some changes or additions to PacketData as we discover breaking changes between the ConstBitStream and PacketData objects.

Specific tests in test_xtcedef.py:

test_string_parameter_parsing
test_integer_parameter_parsing
test_float_parameter_parsing
test_enumerated_parameter_parsing
test_binary_parameter_parsing
test_boolean_parameter_parsing
test_absolute_time_parameter_parsing