test: kill server instance after each test
test: kill server instance after each test

Currently we spawn a new process for the language server for each test.
However, they are all kept alive. This PR fixes the resource leak by killing
the server after each test.

angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service