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.