tests: Enable /BE (EDG) coverage

[StephanTLavavej]

We should enable our test coverage for the /BE compiler option (activating the EDG front-end used for IntelliSense). This is currently disabled by tests/utils/stl/test/tests.py; we'll need additional changes to handle /BE's special behavior. (For example, /BE doesn't perform codegen, so these are inherently compile-only configurations.)

This issue is for EDG imitating MSVC; #704 tracks EDG imitating Clang.

[cbezault]

This isn't disabled. I just add an edg requirement on tests which use the /BE flag. If we're targeting x86 I mark them as enabled in tests/utils/stl/test/config.py.

[StephanTLavavej]

This was implemented by #682.