bpo-33912: Add missing warning filter to test_exec_filename()

[chrisfrederickson]

Added the missing warning filter to test_exec_filename() as mentioned in this issue: https://bugs.python.org/issue33912

First pull request here so let me know if there are any issues (signed the CLA, but it has not been processed yet).

https://bugs.python.org/issue33912

[the-knights-who-say-ni]

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[takluyver]

LGTM

I think this doesn't need a NEWS entry because it's a fix to an unreleased change. I don't know if there's a way to tell the bot that's checking for that that it's not necessary.

[vstinner]

Hi @chrisfrederickson, thank you for your contribution! Did you sign the Python CLA as the Knight told you?
#7839 (comment)

[chrisfrederickson]

@vstinner Yep! And it looks like it has been processed now as the "Contributor Form Received" field says Yes on bus.python.org.

[vstinner]

I ran manually: "./python -W error -m test -v test_warnings" fails without the PR, and pass with this PR.