Fix test 26 on Win XP

Running this test on Windows XP SP3 with Strawberry Perl v5.16.3, I get the following test failure:
```
not ok 26
# Test 26 got: "8" (t\IPC-Run3.t at line 186)
#    Expected: "3"
#  t\IPC-Run3.t line 186 is:     ok -s $fn, 3;
```
Looking at the file test.txt, I see it contains the string ```OUTFFOUT```. I don't yet see *why* this happens, but this proposed change (unlinking the file before the test) fixes the problem...