GitHub: fix GH-261

---------

Co-authored-by: Sutou Kouhei <[email protected]>

GitHub: GH-311

This patch will reduce dependency on `$LOAD_PATH`.

It replaces `require` with `require_relative` for internal library
files. This change is applied only when the replacement can be done
statically and simply.

More advanced replacements are out of scope for this patch, such as:

  * Using `autoload`
  * Changes under `test/`

---------

Co-authored-by: Sutou Kouhei <[email protected]>

GitHub: GH-311

This patch will reduce dependency on `$LOAD_PATH`.

More advanced replacements are out of scope for this patch, such as:

  * Changes under `test/`

---------

Co-authored-by: Sutou Kouhei <[email protected]>

Recently there have been some issues in Ruby head with binstubs. The
following output is from
https://github.com/ruby/ruby-dev-builder/actions/runs/15884945465/job/44794519509:

Like to add `test-unit` to the CLI test, and having a version option
makes for a simple check.

```
Run ruby test_files/cli_test.rb

───── CLI Test ─────────────────
bundle    ✅   2.7.0.dev
gem       ✅   3.7.0.dev
irb       ✅   1.15.2
racc      ✅   1.8.1
rake      ✅   13.3.0
rbs       ❌   missing binstub
rdbg      ❌   missing binstub
rdoc      ✅   6.14.1

ruby 3.5.0dev (2025-06-25T17:13:11Z :detached: a1996b32a9) +PRISM [x86_64-linux]

bad exit
Error: Process completed with exit code 1.
```

Co-authored-by: Sutou Kouhei <[email protected]>

GitHub: fix GH-311

This is a follow-up to GH-312 and the final patch to fix GH-311.
This patch will reduce dependency on `$LOAD_PATH`.

It replaces `require` with `require_relative` for internal library files
in `test/`.

If you want to run tests against the installed `test-unit`, you can
still use the `test/` directory like this:

This example runs tests against the installed `test-unit` version 3.6.8.

```console
$ ruby -I $(gem env home)/gems/test-unit-3.6.8/lib bin/test-unit test
```

GitHub: GH-316