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 test skipped sometimes due to failure to load 'rake/packagetask'.
This is due to manipulation of $LOAD_PATH by other rubygems tests. If
rake is loaded before any rubygems tests run, then it works fine.

To reproduce the skipping behavior:
  $ make test-all TESTOPTS="-j6 --test-order=sorted test/rubygems/test_*.rb"

  Ignoring debug-1.7.1 because its extensions are not built. Try: gem pristine debug --version 1.7.

  Revert "Clear gem paths for each test"

  This reverts commit 6698b580ddad8cfa8c5c86df9328472820d3ee6a.

So that rubygems can find the bundled rake.

This reverts commit 7864efa105921eb3900c843126f2e0db02b9c6ae.

This reverts commit a1c4cab11d613d7df037a81a770ee44a23a2e9be.

http://ci.rvm.jp/results/trunk@ruby-iga/3690333
> tool/lib/test/unit/parallel.rb:68:in `close': Bad file descriptor (Errno::EBADF)