This commit merges the two vectors of specifiers into a single one introducing
the concept of a "TestMode" which is a tri-state enum specifying how a specifier
is to be tested (as documentation, as an executable module or as both).
This is determined during the collection phase and determines how a specifier
will be executed based on how the specifier was collected (directly or not) and
if it has an eligible media_type when fetched.
For example "deno test README.md" is marked as documentation because, while it
is a direct inclusion it is not an executable media type therefore will only
have the fenced code blocks that can be parsed from it tested.
Classic worker scripts are now executed in the context of a Tokio
runtime. This does mean we can not spawn more tokio runtimes in
"op_worker_sync_fetch". We instead spawn a new thread there, that can
create a new Tokio runtime that we can use to block the worker thread.
