Make escaping strings more consistent.

[dnwpark]

When running the migration described in #5728, the output is changed to:

json output:

{
  Json("{\"parent\": \"m1o72ual7w4vvc5xkdfyigfisoeww7ah5ucwyy45kljbwbtwwwh5ua\", \"complete\": true, \"proposed\": null, \"confirmed\": [\"DROP FUTURE nonrecursive_access_policies;\", \"ALTER TYPE default::User RENAME TO default::Foo;\", \"ALTER TYPE default::Foo {\\n    CREATE REQUIRED PROPERTY year_q: std::str {\\n        CREATE CONSTRAINT std::regexp(r'^\\\\d{4}-\\\\d$');\\n    };\\n};\", \"DROP SCALAR TYPE default::PigFamily;\"]}"),
}

default output:

{
  'DROP FUTURE nonrecursive_access_policies;
ALTER TYPE default::User RENAME TO default::Foo;
ALTER TYPE default::Foo {
    CREATE REQUIRED PROPERTY year_q: std::str {
        CREATE CONSTRAINT std::regexp(r\'^\\d{4}-\\d$\');
    };
};
DROP SCALAR TYPE default::PigFamily;',
}

Note: Json output changes as the backslashes are now escaped as well as the quotes. This behaviour should be more "correct".

[msullivan]

We don't want to have to an add unescape_string here. This is testing exposed behavior; if we update this, we'd need to update our CLI's migration flows, and we'd break any user using this.

[msullivan]

I'd prefer not to escape double quotes

[msullivan]

Should we make this table driven?
(Or maybe just not bother)

[msullivan]

Failure's just in one of your new tests