Check RBI syntax in DslCompiler test helper

[bdewater]

Motivation

Prevent #1867 from happening again.

Closes #554 (supersedes it)

Implementation

Sorbet sigs are valid Ruby syntax, so the bad code that aforementioned PR fixes will fail:

% bin/test spec/tapioca/dsl/compilers/active_record_relations_spec.rb
Started with run options --seed 10501

Tapioca
  Dsl
    Compilers
      ActiveRecordRelations
        initialize
          it gathers only ActiveRecord constants with no abstract classes PASS (0.39s)
          it gathers no constants if there are no ActiveRecord classes    PASS (0.38s)

Tapioca
  Dsl
    Compilers
      ActiveRecordRelations
        decorate
          it generates handles composite primary keys                    ERROR (0.76s)
Minitest::UnexpectedError:         SyntaxError: Expected generated RBI file for `Post` to not have any parsing errors.
        
        Got these parsing errors:
        
        -e:55: unexpected token ")" https://srb.help/2001
            55 |    sig { params(args: NilClass, block: T.proc.params(object: ::Post).void)).returns(T.nilable(::Post)) }
                                                                                           ^
        Errors: 1
        
        
            spec/tapioca/dsl/compilers/active_record_relations_spec.rb:1434:in `block (3 levels) in <class:ActiveRecordRelationsSpec>'

          it generates proper relation classes and modules               ERROR (0.57s)
Minitest::UnexpectedError:         SyntaxError: Expected generated RBI file for `Post` to not have any parsing errors.
        
        Got these parsing errors:
        
        -e:55: unexpected token ")" https://srb.help/2001
            55 |    sig { params(args: NilClass, block: T.proc.params(object: ::Post).void)).returns(T.nilable(::Post)) }
                                                                                           ^
        Errors: 1
        
        
            spec/tapioca/dsl/compilers/active_record_relations_spec.rb:738:in `block (3 levels) in <class:ActiveRecordRelationsSpec>'

Tests

Added tests.

[KaanOzkan]

I was thinking of running type checking instead if it's not too slow to cover other potential issues.

[Morriar]

I agree with @KaanOzkan, checking the generated files with Sorbet will provide more value.

You can take a look at this old draft trying to do exactly that: #554.

[Morriar]

See my previous comment.

[KaanOzkan]

Thanks

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