Add tests with multiple color attachments without export

Add tests with multiple color attachments without export

These new tests are to verify if it works correctly when some of color
attachments are not exported in FS.

This also includes a tiny refactor to reuse testDifferentAttachmentSizes
since it's using multiple render targets in FS.

- testDifferentAttachmentSizes -> testMultiAttachments
- makeRenderPassDifferentAttachmentSizes -> makeRenderPassMultiAttachments

New tests:
dEQP-VK.pipeline.*.framebuffer_attachment.multi_attachments_not_exported*

Affects:
dEQP-VK.pipeline.*.framebuffer_attachment.*

VK-GL-CTS Issue: 3927

Components: Vulkan
Change-Id: I863e29c7c7640ee024fc098c31e41fcddf6086c0