Fix wrong initial Grid row height when autoHeight is enabled

When autoHeight is enabled, Grid should be in the DOM before grid row
layouting (row height calculation). Otherwise, element outerHeight
always returns 0, which leads to wrong row height calculation (see
GridRow#_computeAutoHeight).

Render all items again after Grid is added to the DOM in case of
autoHeight.

453711: [Nebula Grid] Wrong initial line height
https://bugs.eclipse.org/bugs/show_bug.cgi?id=453711

Change-Id: I2669237048d5d17eef41ce0fe7cbb9f9c06656d5
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>
Extract concurrency helpers from test fixture

The purpose of the test fixture is to emulate the RAP environment.
These methods do not belong to the fixture.

Change-Id: I782ce6f0205842bf6b61a5f23d0228315e488c86
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>
Extract serialization helpers from test fixture

The purpose of the test fixture is to emulate the RAP environment.
These methods do not belong to the fixture.

Change-Id: I2e633a3db22208087f88c8d512f2f9a807cf43c3
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>