tests/README: fix example about `make -e' usage

tests/README: fix example about `make -e' usage

* tests/README (Section "Writing test cases" subsection "Do"): When
some variable is never initialized in the Makefile, `-e' is not
necessary in order to override it.  DESTDIR is such a variable: we
ensure that we do not ever initialize it.  And as such, it is quite
portable to use:
  $ make DESTDIR=/foo/bar install
and in fact, quite widely used.
So our example about when `make -e' is required, which references
the `DESTDIR' variable, is poorly chosen, if not downright wrong.
Rewrite it to use `prefix' as the overridden variable instead.