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, 4 insertions, 4 deletions

diff --git a/tests/mv/childproof b/tests/mv/childproof
index 04774d294..efe7225d6 100755
--- a/tests/mv/childproof
+++ b/tests/mv/childproof
@@ -32,7 +32,7 @@ cp a/f b/f c 2> /dev/null && fail=1
 test -f a/f || fail=1
 test -f b/f || fail=1
 test -f c/f || fail=1
-test "`cat c/f`" = a || fail=1
+test "$(cat c/f)" = a || fail=1
 rm -f c/f
 
 # With --backup=numbered, it should succeed
@@ -47,7 +47,7 @@ mv a/f b/f c 2> /dev/null && fail=1
 test -f a/f && fail=1
 test -f b/f || fail=1
 test -f c/f || fail=1
-test "`cat c/f`" = a || fail=1
+test "$(cat c/f)" = a || fail=1
 
 # Make sure mv still works when moving hard links.
 # This is where the same_file test is necessary, and why
@@ -76,8 +76,8 @@ echo a > a/f || fail=1
 echo b > b/f || fail=1
 ln -f a/f b/f c 2> /dev/null && fail=1
 # a/f and c/f must be linked
-test `stat --format %i a/f` = `stat --format %i c/f` || fail=1
+test $(stat --format %i a/f) = $(stat --format %i c/f) || fail=1
 # b/f and c/f must not be linked
-test `stat --format %i b/f` = `stat --format %i c/f` && fail=1
+test $(stat --format %i b/f) = $(stat --format %i c/f) && fail=1
 
 Exit $fail