There might be code out there that expect `ruby -v` to print only one
line. Since MJIT shows +JIT in `ruby -v` and RUBY_DESCRIPTION, let's
show +YJIT.

The crash report doesn't show anything about MJIT, so adjust the test.
The "test_ruby_version" test was unaware of RUBY_YJIT_ENABLE and so
was failing when the variable is set and inherited into the children
processes it spawns. Explicitly unset the variable in the test.