Stop running tests that always fail on CI

[ksss]

The test_all_gems CI job keeps failing for three gems (example run). A CI that always fails is not healthy, so this PR stops running these tests by renaming _test to _test_not_run. bin/test already skips a gem when it has no _test directory, so bin/test itself is unchanged.

This PR does not change any RBS files. Directories starting with _ are not installed by rbs collection, so users are not affected.

gem	reason
minitest/5.25	minitest 5.25.x requires Ruby < 4.0, so bundle lock fails on Ruby 4.0. This can never pass on CI.
mini_magick/5.0	Steep now reports Ruby::RedundantIgnoreComment for 4 # steep:ignore comments in _test/readme.rb.
sps_king/0.4.0	RBS now reports RBS::InstanceVariableTypeError because attr_accessor in subclasses (CreditTransfer, DirectDebit, DirectDebitTransaction) declares instance variable types that differ from the parent class.

@aki19035vc (mini_magick) @casaper (sps_king)
To re-enable your tests, fix the errors above and rename _test_not_run back to _test in a PR.

🤖 Generated with Claude Code

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sps_king

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