Add tcu::just() that wraps argument in to tcu::Maybe.
Add tcu::just() that wraps argument in to tcu::Maybe.

Sometimes relying on implicit conversions is not enough and writing
complete constructor of tcu::Maybe<T> is tedious.

Change-Id: I945ada922e6507152f30109773425401cefd54a3

KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,chadversary/deqp,chadversary/deqp,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,maurossi/deqp,geekboxzone/mmallow_external_deqp,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,maurossi/deqp,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,maurossi/deqp,KhronosGroup/Vulkan-CTS,chadversary/deqp,geekboxzone/mmallow_external_deqp,chadversary/deqp,KhronosGroup/Vulkan-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/Vulkan-CTS,chadversary/deqp,chadversary/deqp,geekboxzone/mmallow_external_deqp,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,maurossi/deqp,googlestadia/VK-GL-CTS,maurossi/deqp,maurossi/deqp,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/VK-GL-CTS