Add more extensive coverage for Task.Timeout

[nirinchev]

Description

Noticed coverage was quite lacking there and while it's not particularly complex functionality, figured it's worth adding a simple test to ensure it works as intended.

[coveralls]

Pull Request Test Coverage Report for Build 5621386565

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 20 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 82.759%

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Files with Coverage Reduction	New Missed Lines	%
Realm/Realm/Sync/AppConfiguration.cs	1	92.59%
Realm/Realm/Sync/App.cs	19	73.86%

Totals
Change from base Build 5614974815:	0.1%
Covered Lines:	6168
Relevant Lines:	7317

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💛 - Coveralls