build: enable tests by default if nunit dep is found (bgo#690010)

Instead of disabling tests by default (which is not a good habit
towards newcomers), or the too optimistic approach of enabling
them by default, there can be something in between: enable them
by default if the NUnit dependency is found (allowing the flag
--disable-tests too), and disable them by default if they are
not found (allowing --enable-tests flag, which would fail by
informing the user about what is the dependency and version
required).