Test preamble: unify chdir 't' if -d 't';
Test preamble: unify to dot slash test dot pl
Test preamble: if requiring ./test.pl, needs chdir.

Exception: t/op/chdir, which does things its own way, for obvious reasons.
Test preamble: explicit @INC, instead of unshift.

Exception: t/run/switchI.t, needs the unshift way.