maint.mk: improve sc_cross_check_PATH_usage_in_tests

maint.mk: improve sc_cross_check_PATH_usage_in_tests

With my recent change in init.sh from the two-line form:
    -#   : ${srcdir=.}
    -#   . "$srcdir/init.sh"; path_prepend_ .
    +#   . "${srcdir=.}/init.sh"; path_prepend_ .
I noticed that using the one-line form would cause this test
to fail with a false-positive, or to stop working altogether,
depending on whether help-version changed or all the tests did.
* top/maint.mk (_hv_regex): Remove this definition.
(_hv_regex_weak): Use a weak regex to select all init.sh-sourcing files.
(_hv_regex_strong): Use a stronger regex to check for conformance.
(sc_cross_check_PATH_usage_in_tests): Rewrite to use the above.
Give a separate diagnostic for lack of conforming use.