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.
I think it's redundant - if the latter one passes, then it means that both parsing and stringifing work well.