tests: fix ATF_TESTS_PYTEST no clean build

The hack with .xtmp file was effectively making the make to
ignore changes to the sources, breaking NO_CLEAN builds. The
hack can be actually omitted as setting SCRIPTSNAME_${_T} for
every test is sufficient to prevent renaming by bsd.prog.mk.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D42283