Test suite updates for the @SHELL@ change:
* exsource.test: Set SHELL on `make' command line.
* ansi.test: Set SHELL on `make' command line.
* texinfo.test: Set SHELL on `make' command line.
* insh2.test: Set SHELL on `make' command line.
* empty.test: Rewrite @SHELL@ to /bin/sh.