Unit tests coverage increased to> 90%

[gpavlovych]

Coverage of the following classes described in #38 increased to have >90%:

• ExplorerItemViewModel 90.58%
• ExplorerViewModel 91.20%
• EnvironmentViewModel 94.13%
• SingleEnvironmentExplorerViewModel 91.30%
• DeploySourceExplorerViewModel 91.34%
• DeployDestinationViewModel 100%

Coverage of the following classes described in #40 increased to have >90%:

• MenuViewModel 100.00%
• ToolboxCategoryViewModel 100.00%
• ToolDescriptorViewModel 93.90%
• ToolboxViewModel 100.00%
• ConnectControlViewModel 99.46%
• HelpDescriptorViewModel 100.00%
• HelpModel 100.00%
• HelpWindowViewModel 97.92%
• ManageDatabaseSourceModel 100.00%
• ManageDatabaseSourceViewModel 93.28%
• ManageEmailSourceModel 100.00%
• ManageEmailSourceViewModel 94.37%
• ManageNewServerSourceModel 100.00%
• ManageNewServerViewModel 95.78%
• SharepointServerSourceModel 100.00%
• SharepointServerSourceViewModel 95.62%
• ManageWebServiceSourceModel 100.00%
• ManageWebServiceSourceViewModel 98.40%
• ManagePluginSourceModel 100.00%
• ManagePluginSourceViewModel 96.57%
• SourceBaseImpl 92.16%
• MessageBoxViewModel 95.92%
• EmailAttachmentVm 100.00%
• SplashViewModel 96.13%