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])}
                                         ^^^^^^^^^^
```

This change prevents default gems from inadvertently depending on
bundled gems. This issue was discovered by being able to
`require "rbs"` from test/irb.

Changed since 55bf0ef1aa7c936b564b883196de1ace4be4cc7e.

As `make test-tool` does not use gems, and no Rubygems stuffs is
needed, so moved such things to test/runner.rb.  Also no longer
nees `--test-target-dir` option.

tool/test/runner.rb had been copied from test/runner.rb.
test/runner.rb was for `make test-all`, and tool/test/runner.rb was for
`make test-testframework` and `make test-tool`.
But I want to avoid the code clones.

This change makes tool/test/runner.rb support --test-target-dir option
which allows tool/test/runner.rb to run `make test-all`.
Now we can remove test/runner.rb.

  ref. c3c0e3f5c9444c197779cb242de46dfffda79dec