tests: Merge Encode and Decode tests data

Previously there were two separate tables - for decode and encode tests.

The table for encode tests was very small. TestDecode, which was
operating on data from decode table, was also performing checks that
decode(encode(object)) is idempotent - the work which is already too by
TestEncode. However the coverage of input objects from decode table was
not a strict superset of encode table objects.

For the reasons above let's stop this divergence. Let's have a common
table that define tests where for every test case there can be:

	- an "in" object,
	- a pickle, and
	- an "out" object

where

	1. pickle must decode to "out" object, and
	2. encoding "in" object must give some pickle that decodes to "out" object.

	NOTE: In the usual case "in" object == "out" object and they can only
	differ if "in" object contains a Go struct.

This will allow us to cover all existing decode and encode tests logic.

However the coverage of each logic is now higher - for example Encoder
tests are now run on every object from main table, not only for 3 cases
like it was before.