This introduces a new `ServoTestUtils` namespace with a `forceLayout()`
function. This `forceLayout()` function returns an object which
currently has an array of the phases ran by layout. Also moves
`panic()`, `crashHard()` and `advanceClock()` from `TestBinding` to this
new namespace. `TestBinding` is meant to test the bindings generation,
but these are more generic helpers for Servo-only tests.
Testing: This change adds a series of Servo-only tests that verify the
behavior of the new `forceLayout()` function. Tests that rely on the
moved interfaces are updated and should continue to pass.
Signed-off-by: Luke Warlow <lwarlow@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>
