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, 6 insertions, 6 deletions

diff --git a/tests/misc/ls-time b/tests/misc/ls-time
index a99829480..7bc3c642b 100755
--- a/tests/misc/ls-time
+++ b/tests/misc/ls-time
@@ -47,7 +47,7 @@ touch -a -d "$u1" a || framework_failure_
 
 
 # A has ctime more recent than C.
-set `ls -c a c`
+set $(ls -c a c)
 test "$*" = 'a c' || fail=1
 
 # Sleep so long in an attempt to avoid spurious failures
@@ -58,7 +58,7 @@ sleep 2
 ln c d || framework_failure_
 
 # Before we go any further, verify that touch's -m option works.
-set -- `ls --full -l a`
+set -- $(ls --full -l a)
 case "$*" in
   *" $t3:00.000000000 +0000 a") ;;
   *)
@@ -77,7 +77,7 @@ EOF
 esac
 
 # Ensure that touch's -a option works.
-set -- `ls --full -lu a`
+set -- $(ls --full -lu a)
 case "$*" in
   *" $u1:00.000000000 +0000 a") ;;
   *)
@@ -94,16 +94,16 @@ EOF
   ;;
 esac
 
-set `ls -ut a b c`
+set $(ls -ut a b c)
 test "$*" = 'c b a' && : || fail=1
 test $fail = 1 && ls -l --full-time --time=access a b c
 
-set `ls -t a b c`
+set $(ls -t a b c)
 test "$*" = 'a b c' && : || fail=1
 test $fail = 1 && ls -l --full-time a b c
 
 # Now, C should have ctime more recent than A.
-set `ls -ct a c`
+set $(ls -ct a c)
 if test "$*" = 'c a'; then
   : ok
 else