Automatic smoke test

[hacst]

Issues like #2837 make it clear that we need automatic tests to ensure Mumble keeps working on legacy platforms.

We should setup smoke tests on at least Windows XP and Windows 7 for all our snapshots and releases. These - for now - would include:

• Installing the msi
• Launching the client

A possible extension could be:

• Install msi client only
• Launch the client
• Install msi with murmur only
• Launch murmur
• Install msi with both
• Launch murmur
• Launch mumble
• Connect mumble to local murmur

Going further we should have some actual automated tests to verify basic client functionality like audio sending and reception.

Open questions:

• What infrastructure can we use for this task
• Should it be a blocking part of our normal release pipeline or something we try to track during development
• ...

[mkrautz]

Also, Windows 10 Insider builds would be nice.