Doc: QtTest - Fix documentation for TestCase and SignalSpy

The property/methods documentation needs to be available in the
components qml file. Thus we now import those components from the
qml files directly instead of using fake header files.

Rename the Qt Quick Test page to Qt Quick Test Qml Types to be
consistent with the other submodules.

Task-number: QTBUG-33587

Change-Id: Ifb1df8e7d15f2e23b4b4268e5df138934e62fc42
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>