I have seen this too just running "make THREADS=8". Looks like it's because the other tests in this directory are cleaning too aggressively. From the Makefile:

...
clean:
    rm -f *.o *.hi

T6145: clean
    '$(TEST_HC)' $(TEST_HC_OPTS) --make -v0 -package ghc T6145
    ./T6145 "`'$(TEST_HC)' $(TEST_HC_OPTS) --print-libdir | tr -d '\r'`"
...