First, documented the existing `CTEST_DISABLE_$(TEST_GROUP)` pattern
for conditionally disabling tests based on missing host features.
Added variant of above, `CTEST_DISABLE_NONSELFHOST_$(TEST_GROUP)`,
which is only queried in contexts where the target is not on the
CFG_HOST list (which I interpret as the list of targets that our host
can compatibly emulate; e.g. the example that i686 and x86_64 can in
theory run each others' tests).
Driveby fix: Remove redundant copy of
check-stage$(1)-T-$(2)-H-$(3)-$(4)-exec dependency declaration.
4edf7b8c34