Fix regression in colon{5,6}.test (failures on AIX 5.3).

Fix regression in colon{5,6}.test (failures on AIX 5.3).

* tests/colon5.test: Also substitute `@SHELL@' with `$SHELL' when
post-processing the generated Makefile.in, to work around a bug
of AIX 5.3 make which doesn't allow setting the `$(SHELL)' macro
on the commend line.  Calls to `$MAKE' adjusted accordingly.
* tests/colon6.test: Likewise.

Regression introduced in commit v1.11-175-gf9fe878 "Modernize,
improve and/or extend tests `colon*.test", and reported by Ralf
Wildenhues.