stop using backticks in tests/

[Jim Meyering]

IMHO, bourne shell backticks `...` have been anachronistic for some time,
and we've arranged with gnulib's init.sh so that all of coreutils' tests
can safely use $(...) instead.   That has covered most new tests.
This series removes/replaces all legacy `...` uses.

There are a few remaining in init.sh and check.mk, and
one in the stdbuf test, but that's it.
I'll add a syntax-check to keep it that way.

[PATCH 1/4] tests: convert one `...` expression manually
[PATCH 2/4] tests: convert nearly all `...` expressions to $(...)
[PATCH 3/4] tests: use perl's qx!...! rather than `...`
[PATCH 4/4] tests: remove nearly all remaining backticks