test_check_slow_queries_digest_result: fix filename

this was not html.

Also use os.path.join to manipulate paths
19 jobs for <span class="ref-name">fix/erp5-slow-queries-logs</span> in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:30:39

failed SlapOS.Eggs.UnitTest-Jerome.Python3

00:30:36

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:15:54

failed SlapOS.Eggs.UnitTest-Jerome.Python2

01:04:45

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:13:19

failed SlapOS.Eggs.UnitTest-Jerome.Python2

failed SlapOS.Eggs.UnitTest-Jerome.Python2

00:17:47

passed SlapOS.Eggs.UnitTest-Jerome.Python2

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:24:18

failed SlapOS.Eggs.UnitTest-Jerome.Python2

failed SlapOS.Eggs.UnitTest-Jerome.Python2

00:16:52

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:12:48

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:16:18

passed SlapOS.Eggs.UnitTest-Jerome.Python3

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:18:54

failed SlapOS.Eggs.UnitTest-Jerome.Python3

00:36:12

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:17:06

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:21:19

passed SlapOS.Eggs.UnitTest-Jerome.Python3