tests: use "Exit $fail", not (exit $fail); exit $fail

* tests/test-lib.sh (Exit): New function by Ralf Wildenhues in automake
http://git.sv.gnu.org/gitweb/?p=automake.git;a=commitdiff;h=20594c08f63
* tests/**: Convert all uses:

This restrictive change converted the vast majority:

  git grep -l '^(exit \$fail); exit \$fail$' \
    | xargs perl -pi -e 's/'^\(exit \$fail\); exit \$fail$/Exit \$fail/'

And this did the rest, plus a few undesirable ones, so I manually
backed out the changes to ChangeLog-* and build-aux/check.mk:

  git grep -l -E '\(exit [^)]+\); exit ' \
    | xargs perl -pi -e 's/\(exit (.+?)\); exit \1/Exit $1/'

tests/du/basic

@@ -91,4 +91,4 @@ EOF
   compare out exp || fail=1
 fi
 
-(exit $fail); exit $fail
+Exit $fail