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/du/2g b/tests/du/2g
index a901ddbe9..96cf62d7c 100755
--- a/tests/du/2g
+++ b/tests/du/2g
@@ -30,7 +30,7 @@ very_expensive_
 
 # This technique relies on the fact that the 'Available' kilobyte
 # count is the number just before the one with a trailing '%'.
-free_kb=`df -kP .|tail -1|sed 's/ [0-9][0-9]*%.*//;s/ *$//;s/.* //'`
+free_kb=$(df -kP .|tail -1|sed 's/ [0-9][0-9]*%.*//;s/ *$//;s/.* //')
 case "$free_kb" in
   [0-9]*) ;;
   *) skip_ "invalid size from df: $free_kb";;
@@ -47,7 +47,7 @@ test $min_kb -lt $free_kb ||
 big=big
 rm -f $big
 test -t 1 || printf 'creating a 2GB file...\n'
-for i in `seq 100`; do
+for i in $(seq 100); do
   # Note: 2147483648 == 2^31. Print floor(2^31/100) per iteration.
   printf %21474836s x >> $big || fail=1
   # On the final iteration, append the remaining 48 bytes.