Only use nested assertion syntax

[okolesnyk]

Value

• Streamlined test writing experience

AC:

• MFTF only accepts nested assertion syntax:

<assertInstanceOf stepKey="assertInstanceOf" message="pass">
   <expectedResult type="const">User::class</expectedResult>
   <actualResult type="variable">text</actualResult>
</assertInstanceOf>

• Schema does not allow declaration of old assertion syntax:

<assertNotEquals expected="{$grabTotalBefore}" expectedType="string" actual="{$grabTotalAfter}" actualType="string"  stepKey="assertNotEquals"/>

• Upgrade script added to bin/mftf upgrade
• CE/EE/B2B/etc PRs can be done before MFTF 3.0.0 release