Make sure the PWD environment variable points to the t directory.

Make sure the PWD environment variable points to the t directory.

runtests is typically run under /bin/sh.  If the user uses a different
interactive shell (such as /bin/ksh) that maintains the PWD environment
variable, but /bin/sh does not, then the 'cd t' line in runtests ends
up changing the working directory without updating $PWD.  Several tests
in t/io/fs.t rely on being able to change directories and then get back
to the original.  The tests assume that if $PWD is set at all, then
it is set correctly.  This fix changes runtests to ensure it is so.