The README says:

Currently all tests which use the /BE or /clr:pure options are unsupported.

This is outdated. We added support for the /BE option a long time ago. On the other hand, we marked /clr (in addition to /clr:pure) as unsupported because we don't build the relevant support machinery yet.

The simplest fix is to edit this sentence of the README, replacing /BE with /clr (so this is a "good first issue").

Extra credit: We could add an additional sentence, saying "The /BE option is unsupported for x64." This isn't strictly necessary, but it would explain the biggest remaining source of UNSUPPORTED tests.

Thanks to @d-winsor for noticing this.

(Aside: The undocumented compiler option /BE activates the EDG compiler front-end used for IntelliSense, and is unsupported in general. However, it is supported for the specific purpose of testing the STL.)