tests: Avoid check-subst failures in QEMU user-mode environments.

* Makefile.devel (GNULIB_MODULES): Add stdbool.
(gnulib-imported-files): Copy also tests/qemu.h.
* tests/is-native.c: New file.
* tests/Makefile.in (is-native, is-native.@OBJEXT@): New targets.
(clean): Remove the 'is-native' program.
(SOURCE_FILES): Add is-native.c.
(IMPORTED_FILES): Add qemu.h.
* tests/check-subst: Invoke 'locale charmap' only in native
environments.