tests: shell: Fix skip checks with --host mode

tests: shell: Fix skip checks with --host mode

When testing host binaries, XT_MULTI variable contains just the program
name without path component which most skip checks didn't expect. Fix
them, and while being at it also reduce indenting level in two scripts
by moving the skip check up front with an early exit call.

Fixes: 416898e335322 ("tests/shell: Support testing host binaries")
Signed-off-by: Phil Sutter <phil@nwl.cc>