fixed decimal value generation, incremented MaxTests prop to 10000

The issue was related to decimal precision, my code was generating too long decimals. I switched to the default FsCheck decimal generation. I also increased MaxTests property up to 10000, so that the test runs are more deterministic, and removed the other property test as the latter should be now exhaustive.
Probably yes, but is this a common use case to create JsonValue by hand, not by parsing a json string? Anyway I can bring this test back if you like.