Create subdirectory t/opbasic. Move 5 test files there.

Create subdirectory t/opbasic.  Move 5 test files there.

t/opbasic will hold files formerly held in t/op but which, unlike the vast
majority of tests in the latter directory, are ineligible to use t/test.pl as
a source of test functions.  Affected files:

arith.t
cmp.t
concat.t
magic_phase.t
qq.t

This commit does nothing more than create the new subdirectory and move the
files into it.

For: RT #115838