[nth-check] simplified a > 1, fixed a < -1
[tests] added tests for nth-check

the first self-written tests this module contains
[nth-check] simplified code a bit
[tests/sizzle] commented out several tests failing due to parser issues, enabled nth-child tests

the nth-child tests were updated to use an ID selector instead of
`:first`, which won't be supported by CSSselect
[tests] enabled sizzle tests