Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

It was renamed from test_mjit, but we did not maintain it as test_rjit.
We test RJIT very differently.

test/runner.rb and tool/lib/test/unit/parallel.rb must use the same
settings. However, some settings were copied and pasted, while some were
added only to test/runner.rb.
This changeset creates tool/test/init.rb for all settings of test-unit,
which is loaded not only by test/runner.rb but also
tool/lib/test/unit/parallel.rb.

Background: the GEM_HOME environment variable was removed in
test/runner.rb, which prohibit `require "rake"` (note that rake is a
bundled gem). However the parallel mode didn't refrect this setting,
i.e., `require "rake"` was allowed.
This leads to an inconsistency, which actually affected a test test
defines s test class *only when* `require "rake"` is successful.
(test/rubygems/test_gem_package_task.rb)

https://github.com/ruby/ruby/actions/runs/6807729617/job/18511055636#step:8:1714
```
  /home/runner/work/ruby/ruby/src/tool/lib/test/unit.rb:729:in `const_get': uninitialized constant TestGemPackageTask (NameError)

                suites.map! {|r| ::Object.const_get(r[:testcase])}
                                         ^^^^^^^^^^
```

to @__passed__, @@__current__.
@passed is redefined in a few test suites, and this could lead to bugs.

Also rename @options (Runner#options) to @__runner_options__, which is
only used in make test-tool anyway.