tests: adjust quoting in expected output to match new gnulib policy

tests: adjust quoting in expected output to match new gnulib policy

* tests/misc/xstrtol: Use '...' to match new quoting in most places.
However, leave `9x' to match the sole comparison against output
from the quote function, which still uses `...'.
* tests/misc/sort-merge: Likewise, though here I had to leave
`...'-quoted output to match output from four tests.
* tests/pr/pr-tests: Convert a single `...' to '...'.
* gnulib: Update submodule to latest.