test: migrate to minitest

This commit migrates the test suite to minitest, including
changing the related cmake code accordingly. It adds a typelib/test
for common test file setup, which also allows to load pry and code
coverage as well as making sure that the typelib plugins are not
loaded during tests.
ruby: fix some trivial warnings issued by ruby -w
test: we can use #new_instances to test for #initialize
test: fix test from #from_buffer since we now act on an instance, not a class
ruby: make sure we don't load typelib plugins when initializing the base classes

We are creating null types, which means creating a registry object.
However, Registry#initialize loads plugins if we don't explicitly
disable it, we therefore need to pass false as argument to
Registry#initialize
ruby: using Ruby's built-in support to define Infinity and NaN
ruby,test: keep the test output clean by removing the warnings about method clashes
ruby: fix exception raised by NumericType#from_ruby

Typelib.from_ruby must raise UnknownConversionRequested when
the conversion is not possible. Make sure that NumericType#from_ruby
does so as well.