tests/example/check: Better handling of makefile updates

Rather than checking just for presence of a string in a templated
output file, also have the input file define a target which we try to
run.  This checks that make actually reread the file.

Also add ||false to the grep lines, which makes the stderr output
more obviously contain a failure when the test fails.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>