Fix test_loaded_gem_types faling in test-bundled-gems (take 3)

[tompng]

#46 and #48 was both insufficient.

When running test-bundled-gems with make test-bundled-gems, "prism" does not appear in Gem.loaded_specs.

require 'prism'; require 'rbs'
Gem.loaded_specs.keys #=> ['rbs']

When running test with tool/test-bundled-gems.rb, this does not happen.

require 'prism'; require 'rbs'
Gem.loaded_specs.keys #=> ['prism', 'rbs']

So, we need to avoid using Prism as a loaded gem test (#46) and also need to omit test when RBS's gem_dir does not have sig directory (#48).
This pull request is a mix of #46 and #48.

Additional info

It's because prism is a default gem. Same for irb.

[build]% ./exe/ruby -e "require 'prism'; require 'irb'; require 'rbs'; p Gem.loaded_specs.keys"
["rbs"]

[build]% ./ruby -e "require 'prism'; require 'irb'; require 'rbs'; p Gem.loaded_specs.keys"
["error_highlight", "did_you_mean", "syntax_suggest", "prism", "io-console", "reline", "stringio", "date", "psych", "rdoc", "irb", "forwardable", "tempfile", "delegate", "fileutils", "tmpdir", "io-wait", "singleton", "pathname", "shellwords", "rbs", "set", "json", "prettyprint", "pp", "logger", "tsort", "yaml", "digest", "open3", "find"]