Deprecate loadFixture function

[achingbrain]

It'd be create if we could just fs.readFileSync at the top of tests and let babel-plugin-static-fs or similar figure it out for the browser.

Not sure why we felt the need to invent our own version.

[achingbrain]

There is this idea of loading fixtures from dependencies, as in:

loadFixture('test/foo/bar.txt', 'some-dep')

but see also: require.resolve

fs.readFileSync(require.resolve('some-dep/test/foo/bar.txt'))