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, 18 insertions, 0 deletions

diff --git a/tool/test/init.rb b/tool/test/init.rb
new file mode 100644
index 0000000000..3a1143d01d
--- /dev/null
+++ b/tool/test/init.rb
@@ -0,0 +1,18 @@
+# This file includes the settings for "make test-all".
+# Note that this file is loaded not only by test/runner.rb but also by tool/lib/test/unit/parallel.rb.
+
+ENV["GEM_SKIP"] = ENV["GEM_HOME"] = ENV["GEM_PATH"] = "".freeze
+ENV.delete("RUBY_CODESIGN")
+
+Warning[:experimental] = false
+
+$LOAD_PATH.unshift File.expand_path("../lib", __dir__)
+
+require 'test/unit'
+
+require "profile_test_all" if ENV.key?('RUBY_TEST_ALL_PROFILE')
+require "tracepointchecker"
+require "zombie_hunter"
+require "iseq_loader_checker"
+require "gc_checker"
+require_relative "../test-coverage.rb" if ENV.key?('COVERAGE')