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, 5 insertions, 5 deletions

diff --git a/tests/misc/selinux b/tests/misc/selinux
index 810996c87..12ae15f4e 100755
--- a/tests/misc/selinux
+++ b/tests/misc/selinux
@@ -37,23 +37,23 @@ chcon $ctx f d p ||
 
 # inspect that context with both ls -Z and stat.
 for i in d f p; do
-  c=`ls -dogZ $i|cut -d' ' -f3`; test x$c = x$ctx || fail=1
-  c=`stat --printf %C $i`; test x$c = x$ctx || fail=1
+  c=$(ls -dogZ $i|cut -d' ' -f3); test x$c = x$ctx || fail=1
+  c=$(stat --printf %C $i); test x$c = x$ctx || fail=1
 done
 
 # ensure that ls -l output includes the ".".
-c=`ls -l f|cut -c11`; test "$c" = . || fail=1
+c=$(ls -l f|cut -c11); test "$c" = . || fail=1
 
 # Copy each to a new directory and ensure that context is preserved.
 cp -r --preserve=all d f p s1 || fail=1
 for i in d f p; do
-  c=`stat --printf %C s1/$i`; test x$c = x$ctx || fail=1
+  c=$(stat --printf %C s1/$i); test x$c = x$ctx || fail=1
 done
 
 # Now, move each to a new directory and ensure that context is preserved.
 mv d f p s2 || fail=1
 for i in d f p; do
-  c=`stat --printf %C s2/$i`; test x$c = x$ctx || fail=1
+  c=$(stat --printf %C s2/$i); test x$c = x$ctx || fail=1
 done
 
 Exit $fail