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.
