Add test for forwarding calls to self for `MultiSend` and `MultiSendCallOnly`

[mmv08]

This PR:

• Is an iteration of https://github.com/safe-global/safe-contracts/pull/695
• Adds two straightforward tests that enable owners on a Safe while specifying the to address as 0

[coveralls]

Pull Request Test Coverage Report for Build 6851593137

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.567%

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Totals
Change from base Build 6851285320:	0.0%
Covered Lines:	397
Relevant Lines:	405

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💛 - Coveralls