run-tests: ignore ENOENT failures when removing old .err results
When the same test runs in multiple threads and the previous run was a
failure, the threads can race to delete the error output. This fixes
that.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a