1. Fix a long-standing typo in the makefile: the relevant
   CTEST_NAME here is `rpass-full` (with a dash), not
   `rpass_full`.

2. The rpass-full tests depend on the complete set of target
   libraries.  Therefore, the rpass-full tests need to use
   the dependencies held in the CSREQ-prefixed variable, not
   the TLIBRUSTC_DEFAULT-prefixed variable.

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.