Add nvim_oxi::tests::build()

[noib3]

Problem

Having to manually call cargo b --features=.. before running the tests sucks.

Solution

Create a build() function to be used as the body of a build script that automatically builds the dynamic library to test.

All the tests remain unchanged, and a build.rs is added to the crate housing all the integration tests with the following contents:

// build.rs
fn main() -> Result<(), nvim_oxi::tests::BuildError> {
    nvim_oxi::tests::build()
}