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

When the test source directory path contains symbolic links, that
directory might be placed earlier in the result of
`IRB::InputCompletor.retrieve_gem_and_system_load_path`, than other
directories in the source tree that are expanded to the real path.  In
that case, the test file in "csv" under the test directory of csv gem
instead of the library path will be the first candidate, in
`TestIRB::TestCompletion#test_complete_require_library_name_first`.

```
<"'csv"> expected but was
<"'csv/helper">.
```

Check GC.enable'd status before and after test execution.

Write this checker in gc_checker.rb, it was renamed from
gc_compact_checker.rb.

RUBY_TEST_GC_COMPACT=1 enables GC.compact checker which calls
GC.compact after each test.

parallel test (`make test-all TESTS=-j8`) runs tests on specified
number of processes. However, some test checkers written in
`runner.rb` are not loaded. This fix enable these checkers on
parallel tests.

See also: https://github.com/ruby/ruby/pull/2508

Run test suites explicitly instead of auto-running, to get rid of
failures when simplecov is not installed but COVERAGE is set.

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.