regress: avoid interference from systemwide /etc/ksh.kshrc

ksh, even non-interactive, loads /etc/ksh.kshrc by default. On
some systems this can be a problem, e.g. OpenBSD, which installs a
default /etc/ksh.kshrc which is designed for its version of pdksh.

Quoth sh.1:

    On systems that support a system wide /etc/ksh.kshrc
    initialization file, if the filename generated by the expansion
    of ENV begins with /./ or ././ the system wide initialization
    file will not be executed.

src/cmd/ksh93/tests/shtests,
src/cmd/ksh93/tests/builtins.sh,
src/cmd/ksh93/tests/options.sh,
src/cmd/ksh93/tests/pty.sh:
- Instead of emptying or unsetting ENV, ensure it is exported with
  a default value of /./dev/null so we skip loading the system-wide
  profile and load an empty user profile.
- Where a specific ENV path was required for the tests, prefix it
  with '/.' so it starts with '/./'.

src/cmd/ksh93/tests/shtests

@@ -165,7 +165,8 @@ function valxml
 	return $errors
 }
 
-unset DISPLAY ENV FIGNORE HISTFILE
+unset DISPLAY FIGNORE HISTFILE
+export ENV=/./dev/null
 trap + PIPE # unadvertized -- set SIGPIPE to SIG_DFL #
 
 integer compile=-1 posix=-1 utf8=-1