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/cp/link-preserve b/tests/cp/link-preserve
index 785c9e40a..9ca1705f5 100755
--- a/tests/cp/link-preserve
+++ b/tests/cp/link-preserve
@@ -28,8 +28,8 @@ test -f c/a || framework_failure_
 test -f c/b || framework_failure_
 
 
-a_inode=`ls -i c/a|sed 's,c/.*,,'`
-b_inode=`ls -i c/b|sed 's,c/.*,,'`
+a_inode=$(ls -i c/a|sed 's,c/.*,,')
+b_inode=$(ls -i c/b|sed 's,c/.*,,')
 test "$a_inode" = "$b_inode" || fail=1
 # --------------------------------------
 
@@ -38,8 +38,8 @@ touch a
 ln -s a b
 mkdir c
 cp --preserve=links -R -H a b c
-a_inode=`ls -i c/a|sed 's,c/.*,,'`
-b_inode=`ls -i c/b|sed 's,c/.*,,'`
+a_inode=$(ls -i c/a|sed 's,c/.*,,')
+b_inode=$(ls -i c/b|sed 's,c/.*,,')
 test "$a_inode" = "$b_inode" || fail=1
 # --------------------------------------
 
@@ -47,24 +47,24 @@ test "$a_inode" = "$b_inode" || fail=1
 # and translates to hard-linked a and b in the destination dir.
 rm -rf a b c d; mkdir d; (cd d; touch a; ln -s a b)
 cp --preserve=links -R -L d c
-a_inode=`ls -i c/a|sed 's,c/.*,,'`
-b_inode=`ls -i c/b|sed 's,c/.*,,'`
+a_inode=$(ls -i c/a|sed 's,c/.*,,')
+b_inode=$(ls -i c/b|sed 's,c/.*,,')
 test "$a_inode" = "$b_inode" || fail=1
 # --------------------------------------
 
 # Same as above, but starting with a/b hard linked.
 rm -rf a b c d; mkdir d; (cd d; touch a; ln a b)
 cp --preserve=links -R -L d c
-a_inode=`ls -i c/a|sed 's,c/.*,,'`
-b_inode=`ls -i c/b|sed 's,c/.*,,'`
+a_inode=$(ls -i c/a|sed 's,c/.*,,')
+b_inode=$(ls -i c/b|sed 's,c/.*,,')
 test "$a_inode" = "$b_inode" || fail=1
 # --------------------------------------
 
 # Ensure that --no-preserve=links works.
 rm -rf a b c d; mkdir d; (cd d; touch a; ln a b)
 cp -dR --no-preserve=links d c
-a_inode=`ls -i c/a|sed 's,c/.*,,'`
-b_inode=`ls -i c/b|sed 's,c/.*,,'`
+a_inode=$(ls -i c/a|sed 's,c/.*,,')
+b_inode=$(ls -i c/b|sed 's,c/.*,,')
 test "$a_inode" = "$b_inode" && fail=1
 # --------------------------------------
 
@@ -73,8 +73,8 @@ rm -rf a b c d
 touch a; ln a b
 mkdir c
 cp -d a b c
-a_inode=`ls -i c/a|sed 's,c/.*,,'`
-b_inode=`ls -i c/b|sed 's,c/.*,,'`
+a_inode=$(ls -i c/a|sed 's,c/.*,,')
+b_inode=$(ls -i c/b|sed 's,c/.*,,')
 test "$a_inode" = "$b_inode" || fail=1
 # --------------------------------------
 
@@ -83,7 +83,7 @@ rm -rf a b c d
 touch a; chmod 731 a
 umask 077
 cp -a --no-preserve=mode a b
-mode=`ls -l b|cut -b-10`
+mode=$(ls -l b|cut -b-10)
 test "$mode" = "-rwx------" || fail=1
 umask 022
 # --------------------------------------