Follow-up to #46664.

In #46664, `bin/rails test` _without_ args, `bin/rails test:all`, and
`bin/rails test:system` were made to run the `test:prepare` task.
However `bin/rails test` _with_ args (e.g. `bin/rails test --seed 1234`)
and other `test:*` commands (e.g. `bin/rails test:models`) were not made
to do so.

This commit changes `bin/rails test` to always run `test:prepare` unless
exact tests are specified via path arguments (e.g. `bin/rails test
path/to/test.rb`) or a name pattern (e.g. `bin/rails test -n test_foo`).

This commit also refactors the testing Rake tasks so that all of the
logic regarding which files to load and whether to run `test:prepare` is
contained within the `TestCommand` class.