refactor: simplify tests for api.Generate

[alexandear]

The PR refactor tests for the api.Generate function. Changes:

• Remove generated files graph/generated.go, graph/federation.go. These files are re-generated on every test run.
• Use path/filepath instead of path because filepath uses either forward slashes or backslashes, depending on the operating system.
• Remove redundant args struct.
• Handle errors from os.Getwd and os.Chdir.
• Remove wantErr struct field because it's always nil.
• Use T.Cleanup instead of defer func.

[coveralls]

coverage: 75.167% (-0.03%) from 75.198%
when pulling 72bba78 on alexandear:refactor/test-generate
into 28405ac on 99designs:master.

[StevenACoffman]

Hey thanks! I added the removed files to the .gitignore or people will constantly just be adding them back (even though the cleanup is supposed to delete them, it just happens).