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, 13 insertions, 13 deletions

diff --git a/tests/du/basic b/tests/du/basic
index bb0980eee..d8acf6420 100755
--- a/tests/du/basic
+++ b/tests/du/basic
@@ -29,7 +29,7 @@ printf %4096s x > d/1
 cp d/1 d/sub/2
 
 
-B=`stat --format=%B a/b/F`
+B=$(stat --format=%B a/b/F)
 
 du --block-size=$B -a a > out || fail=1
 echo === >> out
@@ -37,11 +37,11 @@ du --block-size=$B -a -S a >> out || fail=1
 echo === >> out
 du --block-size=$B -s a >> out || fail=1
 
-f=`stat --format=%b a/b/F`
-b=`stat --format=%b a/b`
-a=`stat --format=%b a`
-bf=`expr $b + $f`
-tot=`expr $bf + $a`
+f=$(stat --format=%b a/b/F)
+b=$(stat --format=%b a/b)
+a=$(stat --format=%b a)
+bf=$(expr $b + $f)
+tot=$(expr $bf + $a)
 
 cat <<EOF | sed 's/ *#.*//' > exp
 $f	a/b/F
@@ -65,13 +65,13 @@ if is_local_dir_ .; then
   echo === >> out
   du --block-size=$B -S d | sort -r -k2,2 >> out || fail=1
 
-  t2=`stat --format=%b d/sub/2`
-  ts=`stat --format=%b d/sub`
-  t1=`stat --format=%b d/1`
-  td=`stat --format=%b d`
-  tot=`expr $t1 + $t2 + $ts + $td`
-  d1=`expr $td + $t1`
-  s2=`expr $ts + $t2`
+  t2=$(stat --format=%b d/sub/2)
+  ts=$(stat --format=%b d/sub)
+  t1=$(stat --format=%b d/1)
+  td=$(stat --format=%b d)
+  tot=$(expr $t1 + $t2 + $ts + $td)
+  d1=$(expr $td + $t1)
+  s2=$(expr $ts + $t2)
 
   cat <<EOF | sed 's/ *#.*//' > exp
 $t2	d/sub/2