; Reduce quoting for SELECTOR in 'make -C test' (Bug#31744)

; Reduce quoting for SELECTOR in 'make -C test' (Bug#31744)

Before:

    make -C test SELECTOR='\"foo\"'
    make -C test SELECTOR='(quote (tag :some-tag))'

After:

    make -C test SELECTOR='"foo"'
    make -C test SELECTOR='(tag :some-tag)'

* test/Makefile.in: Use single quotes around the command line call to
ert, this means the user doesn't have to backslash escape double
quotes when writing lisp strings for the selector.  Also wrap the
SELECTOR value in (quote ...) so the user won't have to type it
in (and not get tempted to use the '... reader syntax form which would
now fail to work due to using single quotes around the whole shell
arg).
* test/README: Update instructions accordingly.