testgen: stop copying exile tests and generate output assertions

[webern]

Closes #65
Closes #69
Closes #63
Closes #64

testgen: stop copying exile tests (#65)

Move existing exile test files to the input_data directory and stop
copying them during test generation. This commit has some failing tests
because the tests generated by xtest still exist.

The tests generate by testgen do not yet have a serialization assertion
when an output.xml file is present (#69). Once those assertions are
generated, then the failing tests will be fully replicated by testgen
and can be deleted (along with the xtest crate).

testgen: generate output checks (#69)

For exile test cases that have a .output.xml file, create a function
that asserts correct serialization. Unfortunately we still cannot delete
the tests that were generated by xtest until we generate not-well-formed
tests.

testgen: add not-well-formed tests (#63) (#64)

Adds not-well-formed tests for both exile and W3C. Deletes the tests
that had previously been generated through the xtest dependency. Cleans
up build.rs to no longer generate tests.