Like Brett, I think the long term solution is to segregate
implementation-specific tests into a separate file or subdirectory of
files.  Then the main directory of tests could (and I would like)
constitute an executable definition-by-example for the language.  (To
aid this, I would also like the naming of files and tests and sequencing
of tests within files to reflect the structure of the manual as much as
possible -- and would help to make it so.  Separate patches of course.)

Would alternative implementors prefer to wait or have a *temporary*
addition to test_support?

If something is added, I would give it a leading underscore name and
document it as something probably temporary for alternate
implementations to use until a permanent solution is implemented.