Avert crashes when testing in parallel.

Both t/001-basic.t and what was t/004-nolinenumbers.t were trying to write to
a 't/XSTest.c' file.  When run in parallel, this was causing problems when
TEST_JOBS >= 1 (2 on some boxes, 4 on dromedary).

Since all that t/004-nolinenumbers.t was ever trying to do was to run
process_file() without line numbers -- a case not exercised prior to my
2009-11 refactoring/test additions -- the simplest way to avoid these
problems is to stuff the tests from t/004 into t/001 and delete t/001.

For: RT #119231