Use Minitest module instead of MiniTest compatibility layer

[oturpe]

The Minitest package has long been capitalized like that. The variant MiniTest is retained only as a compatibility layer. Switch to the current module name.

This change is motivated by Fedora, which already uses minitest 5.19. That version requires setting the env variable MT_COMPAT to import the compatibility module, and thus revealed the problem in Fedora build system.