Remove test/stdlib assertions that do not depend on RBS signatures

[ksss]

Tests under test/stdlib exist to detect mismatches between RBS signatures and the actual behavior. The criterion used here: does the test fail when the corresponding RBS definition is broken? If not, it only re-tests Ruby itself, which Ruby's own test suite already does.

This removes or rewrites such assertions so that test/stdlib no longer calls assert, assert_equal, assert_instance_of, assert_kind_of or assert_nil, in preparation for a RuboCop cop that forbids them there.

Changes

File	Change	Verified by breaking RBS
Kernel_test.rb	Remove test_exit! (checks subprocess exit status only)	Passed even with exit!: (Symbol) -> Integer
Kernel_test.rb	Remove assert_equal nil, eval('__dir__'). The nil path cannot go through assert_send_type since the spy is always the caller frame	—
Exception_test.rb	Remove assert_instance_of; the following assert_send_type already checks the argument type	—
OpenSSL_test.rb	encrypt_decrypt never ran (no test_ prefix). Renamed to test_encrypt_decrypt (only coverage of Cipher#padding=) and dropped the round-trip assert_equal	Fails with padding=: (String) -> Integer
Regexp_test.rb	Remove test_TimeoutError, which only checked Ruby's superclass	—
Refinement_test.rb	Replace assert(true) calls with refute_send_type '() -> untyped'. Calling import_methods with modules through the spy crashes the VM (cref_replace_with_duplicated_cref_each_frame: unreachable), so only the no-arg case is checked	Fails with import_methods: (*Module) -> self
global_variables_test.rb	assert_global_type now also checks the value against the RBS global declaration (env.global_decls). Previously it only compared against the type given in the test	Fails when $!/$\ types are changed or $1 is removed
global_variables_test.rb	Fix assert_global_type 'nil', $\ → :$\. It passed the value (nil) instead of the symbol, so eval("") was checked and $\ was never tested	—

Out of scope

assert_raises (e.g. assert_raises X do assert_send_type ... end in uri tests, replaceable with assert_send_type_error) and RBS-independent assert_type usages will be handled in follow-up PRs.

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