Fix 3.4 prototype rb failure

[soutaro]

prototype rb in Ruby 3.4 generates record type for more hash literals, with keys of string literals.

That was because the parser <=3.3 generates LIT node for hash keys.

# In Ruby 3.3
irb(main):001> RubyVM::AbstractSyntaxTree.parse('"a"')
=> (SCOPE@1:0-1:3 tbl: [] args: nil body: (STR@1:0-1:3 "a"))
irb(main):002> RubyVM::AbstractSyntaxTree.parse('{ "a" => 1 }')
=> (SCOPE@1:0-1:12 tbl: [] args: nil body: (HASH@1:0-1:12 (LIST@1:2-1:10 (LIT@1:2-1:5 "a") (LIT@1:9-1:10 1) nil)))

Ruby 3.4 fixed this issue generating STR node for hash keys too and caused test failure.

This PR updates the test to expect a record type, and test if LIT is for a string for <=3.3. This will change the result of rbs prototype rb to generate more record types for hash literals.