Improve test_ast_parsing.py

[montyly]

One issue with our current parsing pytest is that it is not robust against new version of solc for the xfail tests. For example, these tests are not working with 0.7.1 and 0.7.2:

slither/tests/test_ast_parsing.py

Lines 81 to 82 in ae7c410

	"for_0.7.1_legacy",
	"for_0.7.2_legacy",

And was not updated when 0.7.3/0.7.4 were released. So the tests fail, while the script should have expected to not support 0.7.3 and 0.7.4 for this test.

We should have two version of XFAIL: the ones that fail for a specific version, and the ones that fail from a version