Add extra assert in flaky couch_file test
The deletion test is flaky sometimes:
```
[2023-05-31T19:44:32.096Z] File delete tests
[2023-05-31T19:44:32.096Z] enable_database_recovery = false, context = delete
[2023-05-31T19:44:32.096Z] couch_file_tests:424: make_enable_recovery_test_case...ok
[2023-05-31T19:44:32.096Z] couch_file_tests:425: make_enable_recovery_test_case...ok
[2023-05-31T19:44:32.096Z] couch_file_tests:426: make_enable_recovery_test_case...ok
[2023-05-31T19:44:32.096Z] couch_file_tests:427: make_enable_recovery_test_case...*failed*
[2023-05-31T19:44:32.096Z] in function couch_file_tests:'-make_enable_recovery_test_case/3-fun-8-'/2 (test/eunit/couch_file_tests.erl, line 427)
[2023-05-31T19:44:32.096Z] in call from eunit_test:run_testfun/1 (eunit_test.erl, line 71)
[2023-05-31T19:44:32.096Z] in call from eunit_proc:run_test/1 (eunit_proc.erl, line 531)
[2023-05-31T19:44:32.096Z] in call from eunit_proc:with_timeout/3 (eunit_proc.erl, line 356)
[2023-05-31T19:44:32.096Z] in call from eunit_proc:handle_test/2 (eunit_proc.erl, line 514)
[2023-05-31T19:44:32.096Z] in call from eunit_proc:tests_inorder/3 (eunit_proc.erl, line 456)
[2023-05-31T19:44:32.096Z] in call from eunit_proc:with_timeout/3 (eunit_proc.erl, line 346)
[2023-05-31T19:44:32.096Z] in call from eunit_proc:run_group/2 (eunit_proc.erl, line 570)
[2023-05-31T19:44:32.096Z] **error:{assertEqual,[{module,couch_file_tests},
[2023-05-31T19:44:32.096Z] {line,427},
[2023-05-31T19:44:32.096Z] {expression,"length ( DeletedFiles )"},
[2023-05-31T19:44:32.096Z] {expected,1},
[2023-05-31T19:44:32.096Z] {value,0}]}
[2023-05-31T19:44:32.096Z] output:<<"">>
```
It seems the renamed files are not found in the $root/.delete directory, which
is odd as renames are supposed to be atomic. Should we fsync dir entries
themselves? We could also add a sleep, but on first attempt is to add and extra
assert that file:name/2 actually returned an ok not some posix error.
1 file changed
