tests: convert nearly all `...` expressions to $(...)

Exempt init.sh because it runs before we're assured to have a
shell that groks $(...).  Exempt *.mk because "$" would have to
be doubled, and besides, any `...` expression in a .mk file is
almost certainly evaluated before init.sh is run.  Finally, also
exempt the perl-based tests, because perl's `...` cannot be
converted to $(...).  Do that by running this command:

git grep -l '`.*`' tests \
  | grep -Ev 'init\.sh|\.mk$' | xargs grep -Lw perl \
  | xargs perl -pi -e 's/`(.*?)`/\$($1)/g'

One minor fix-up change was required after that, due to how
quoting differs:
diff --git a/tests/chmod/equals b/tests/chmod/equals
-    expected_perms=$(eval 'echo \$expected_'$dest)
+    expected_perms=$(eval 'echo $expected_'$dest)

Another was to make these required quoting adjustments:
diff --git a/tests/misc/stty b/tests/misc/stty
...
-  rev=$(eval echo "\\\$REV_$opt")
+  rev=$(eval echo "\$REV_$opt")
...
-      rev1=$(eval echo "\\\$REV_$opt1")
-      rev2=$(eval echo "\\\$REV_$opt2")
+      rev1=$(eval echo "\$REV_$opt1")
+      rev2=$(eval echo "\$REV_$opt2")

Also, transform two files that were needlessly excluded above:
(both use perl, but are mostly bourne shell)

  perl -pi -e 's/`(.*?)`/\$($1)/g' \
    tests/du/long-from-unreadable tests/init.cfg

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/other-fs-tmpdir b/tests/other-fs-tmpdir
index d3d3a55cd..4bdfe6ff5 100644
--- a/tests/other-fs-tmpdir
+++ b/tests/other-fs-tmpdir
@@ -24,13 +24,13 @@ test "${CANDIDATE_TMP_DIRS+set}" = set \
 
 other_partition_tmpdir=
 
-dot_mount_point=`stat -c %d .`
+dot_mount_point=$(stat -c %d .)
 for d in $CANDIDATE_TMP_DIRS; do
 
   # Skip nonexistent directories.
   test -d "$d" || continue
 
-  d_mount_point=`stat -L -c %d "$d"`
+  d_mount_point=$(stat -L -c %d "$d")
 
   # Same partition?  Skip it.
   test "x$d_mount_point" = "x$dot_mount_point" && continue