Unquoted VSTest settings file path

[jnm2]

I wanted to add a different path VSTest argument and I looked at SettingsFile as a pattern to copy, but I found some issues with its current implementation.
The SettingsFile argument is not quoted. Filenames that contain spaces will cause problems. I've confirmed the expected behavior at the command line.

@gep13 pointed out AppendSwitchQuoted which is more idiomatic than the string.Format that was previously used in this method, so I changed all the switches to use AppendSwitch or AppendSwitchQuoted and that resolved the issue.

Fix is here: #1268

[gep13]

Thanks for noticing and fixing this. It will be released in 0.17.0 of Cake.