tests: more uses of $(...) over `...` for command substitution

tests: more uses of $(...) over `...` for command substitution

Somehow missed by the previous changes.  No big deal, fix them now.

* t/ax/depcomp.sh: Here.
* t/autodist.sh: And here.
* t/autodist-no-duplicate.sh: And here.
* t/autodist-subdir.sh: Ad here.
* t/remake11.sh t/self-check-me.tap: And here.
* t/perf/testsuite-summary.sh: And here.
* t/perf/testsuite-recheck.sh: And here.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>