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, 20 insertions, 20 deletions

diff --git a/tests/readlink/can-e b/tests/readlink/can-e
index 5dc73892d..32b75c595 100755
--- a/tests/readlink/can-e
+++ b/tests/readlink/can-e
@@ -19,7 +19,7 @@
 . "${srcdir=.}/init.sh"; path_prepend_ ../src
 print_ver_ readlink
 
-pwd=`pwd`
+pwd=$(pwd)
 my_pwd=$("$abs_top_builddir/src/pwd")
 tmp=d
 
@@ -38,7 +38,7 @@ cd "$pwd/$tmp/removed" || framework_failure_
 
 # Skip this test if the system doesn't let you remove the working directory.
 if rmdir ../removed 2>/dev/null; then
-  v=`readlink -e .` && fail=1
+  v=$(readlink -e .) && fail=1
   test -z "$v" || fail=1
 fi
 
@@ -46,58 +46,58 @@ cd "$pwd/$tmp" || fail=1
 
 for p in "" "$pwd/$tmp/"; do
 
-  v=`readlink -e "${p}regfile"` || fail=1
+  v=$(readlink -e "${p}regfile") || fail=1
   test "$v" = "$my_pwd/$tmp/regfile" || fail=1
 
-  v=`readlink -e "${p}./regfile/"` && fail=1
+  v=$(readlink -e "${p}./regfile/") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}subdir"` || fail=1
+  v=$(readlink -e "${p}subdir") || fail=1
   test "$v" = "$my_pwd/$tmp/subdir" || fail=1
 
-  v=`readlink -e "${p}./subdir/"` || fail=1
+  v=$(readlink -e "${p}./subdir/") || fail=1
   test "$v" = "$my_pwd/$tmp/subdir" || fail=1
 
-  v=`readlink -e "${p}missing"` && fail=1
+  v=$(readlink -e "${p}missing") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}./missing/"` && fail=1
+  v=$(readlink -e "${p}./missing/") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}link1"` || fail=1
+  v=$(readlink -e "${p}link1") || fail=1
   test "$v" = "$my_pwd/$tmp/regfile" || fail=1
 
-  v=`readlink -e "${p}./link1/"` && fail=1
+  v=$(readlink -e "${p}./link1/") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}link1/more"` && fail=1
+  v=$(readlink -e "${p}link1/more") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}link2"` || fail=1
+  v=$(readlink -e "${p}link2") || fail=1
   test "$v" = "$my_pwd/$tmp/subdir" || fail=1
 
-  v=`readlink -e "${p}./link2/"` || fail=1
+  v=$(readlink -e "${p}./link2/") || fail=1
   test "$v" = "$my_pwd/$tmp/subdir" || fail=1
 
-  v=`readlink -e "${p}link2/more"` && fail=1
+  v=$(readlink -e "${p}link2/more") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}link3"` && fail=1
+  v=$(readlink -e "${p}link3") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}./link3/"` && fail=1
+  v=$(readlink -e "${p}./link3/") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}link3/more"` && fail=1
+  v=$(readlink -e "${p}link3/more") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}link4"` && fail=1
+  v=$(readlink -e "${p}link4") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}./link4/"` && fail=1
+  v=$(readlink -e "${p}./link4/") && fail=1
   test -z "$v" || fail=1
 
-  v=`readlink -e "${p}link4/more"` && fail=1
+  v=$(readlink -e "${p}link4/more") && fail=1
   test -z "$v" || fail=1
 done