test-uforth: filetest.fth raced CPython on a shared file

Found 2026-10-19: test-uforth: FAIL — 1 of 17 corpora differ. In filetest, FN1 R/O OPEN-FILE returned a non-zero ior, then THROW -13.

Cause: the recipe runs the native uforth and the CPython oracle concurrently in the same $(UFORTH_SRC) tree, and filetest.fth creates, reopens and deletes fatest1.txt there. Run one after the other, native and CPython were byte-identical twice. This was not a compiler bug.

Fix: filetest.fth alone runs serially (native, then CPython). The other 16 corpora stay in parallel.