Test: make it easier to add tests

[erezrokah]

At the moment most of our tests share the same site:

cli/tests/dev.test.js

Line 9 in aa12f8a

const sitePath = path.join(__dirname, 'dummy-site')

Or use pre-existing directories of sites:

cli/tests/build.test.js

Line 32 in aa12f8a

test('build command - succeeds', async t => {

This makes the tests hard to follow and also adding new tests requires setting up a site manually.

Ideally we would have a test setup utility that allows configuring a site with different configurations (UI settings, netlify.toml settings, functions, etc).