Compatibility with PostCSS 8

[JLHwung]

Thanks for your PostCSS test utilities!

As you probably know, PostCSS 8 was released a few days ago. It would be great if you could make this utility compatible with the latest version. The creators of the project have even released this migration guide.

Currently running postcss-tape with a plugin using updated API will throw

i.process is not a function

because the testPlugin should be initialized by PostCSS

postcss-tape/src/index.js

Line 45 in 01e92f4

const result = await testPlugin.process(sourceCSS, processOptions, pluginOptions);

--- const result = await testPlugin.process(sourceCSS, processOptions, pluginOptions);
+++ const result = await postcss([testPlugin(pluginOptions)]).process(sourceCSS, processOptions);