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, 3 insertions, 3 deletions

diff --git a/tests/mv/hard-3 b/tests/mv/hard-3
index 9dd8ad592..a62167a07 100755
--- a/tests/mv/hard-3
+++ b/tests/mv/hard-3
@@ -58,12 +58,12 @@ test -f x/b || fail=1
 test -f c || fail=1
 
 # The i-node numbers of a and c must be the same.
-ia=`ls -i a` || fail=1; set x $ia; ia=$2
-ic=`ls -i c` || fail=1; set x $ic; ic=$2
+ia=$(ls -i a) || fail=1; set x $ia; ia=$2
+ic=$(ls -i c) || fail=1; set x $ic; ic=$2
 test "$ia" = "$ic" || fail=1
 
 # The i-node number of x/b must be different.
-ib=`ls -i x/b` || fail=1; set x $ib; ib=$2
+ib=$(ls -i x/b) || fail=1; set x $ib; ib=$2
 test "$ia" = "$ib" && fail=1
 
 Exit $fail