Remove the dependency on test-generator

The test-generator crate seems unmaintained, and it's pulling in obsolete crates that take a long time to compile (syn and friends).

I used it originally as a way to automatically scan a directory full of .svg files, and create test cases from them - because that's what the test code in C looked like.

So, for now, the tests/src/*.rs sources enumerate the test files by hand, but hopefully this is made better with a few macros.