Make strings in tests not frozen

[credbbl]

Future versions of Ruby will default to frozen strings and Ruby 3.4 starts to warn about it. However all the tests rely on modifiable, aka not frozen, strings.

[ekohl]

I'd prefer to deal with frozen strings instead because it could hide items.

For my own references: https://rubyapi.org/4.0/o/string#class-String-label-Modifying lists all methods modifying strings and that lists gsub!.

I'd suggest to replacegsub! with something like:

call = call.gsub('"', '\\\\"')

Having said that, I see in the code gsub! is also used in the code.