[SHRINKDESC-90] Extend test coverage of importers/exporters for SPI module

[bartoszmajsak]

Hi guys,

here's what's in:

• Additional tests for importers and exporters. I added "standalone=yes" property to ?xml header which was missing in the original impl.
• Moved direct usage of XMLUnit to XmlAssert class and introduced additional module test-utils containing this class since it's used by spi and test modules. Assuming that we might have more custom assertions used across the modules such separation is reasonable. I simply didn't want to copy & paste XmlAssert.

[ALRubinger]

Upstream: https://github.com/shrinkwrap/descriptors/commit/03066983868cdac2524b87846c88227c4de208f7