Refactor the settings of test-all out

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

1 files changed, 2 insertions, 8 deletions

diff --git a/tool/lib/test/unit/parallel.rb b/tool/lib/test/unit/parallel.rb
index 407e3fa1a2..f2244ec20a 100644
--- a/tool/lib/test/unit/parallel.rb
+++ b/tool/lib/test/unit/parallel.rb
@@ -1,12 +1,6 @@
 # frozen_string_literal: true
-$LOAD_PATH.unshift "#{__dir__}/../.."
-require_relative '../../test/unit'
-
-require_relative '../../profile_test_all' if ENV.key?('RUBY_TEST_ALL_PROFILE')
-require_relative '../../tracepointchecker'
-require_relative '../../zombie_hunter'
-require_relative '../../iseq_loader_checker'
-require_relative '../../gc_checker'
+
+require_relative "../../../test/init"
 
 module Test
   module Unit