[mono] ToolsetReader: allow app.config to be read when running tests
[mono] ToolsetReader: allow app.config to be read when running tests

This was breaking a test in Engine. The check is not required and now
matches what upstream has. Usage of
`ToolsetConfigurationReaderHelpers.ConfigurationFileMayHaveToolsets`
will get cleaned up when we update and push the target framework fallback
PR upstream.

mono/msbuild,mono/msbuild,mono/msbuild,mono/msbuild