Allow installing and testing with 2019 Preview

Add Visual Studio 2019 Preview to launchSettings.json.
Use updated version of VsixTesting.Installer package.

Author: jcansdale

test/Launcher/InstallAndStart.csproj

@@ -3,7 +3,7 @@
     <TargetFramework>net461</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="VsixTesting.Installer" Version="0.1.36-beta-g2c36f902a0" />
+    <PackageReference Include="VsixTesting.Installer" Version="0.1.38-beta-g6b91b368d9" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\GitHub.VisualStudio\GitHub.VisualStudio.csproj" />

test/Launcher/Properties/launchSettings.json

@@ -11,6 +11,10 @@
     "Visual Studio 2017 - Team Explorer": {
       "executablePath": "$(VsixTestingInstallerPath)",
       "commandLineArgs": "/ApplicationPath \"C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\TeamExplorer\\Common7\\IDE\\devenv.exe\" /RootSuffix Exp /InstallAndStart \"$(SolutionDir)\\build\\$(Configuration)\\GitHub.VisualStudio.vsix\""
+    },
+    "Visual Studio 2019 - Preview": {
+      "executablePath": "$(VsixTestingInstallerPath)",
+      "commandLineArgs": "/ApplicationPath \"C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\devenv.exe\" /RootSuffix Exp /InstallAndStart \"$(SolutionDir)\\build\\$(Configuration)\\GitHub.VisualStudio.vsix\""
     }
   }
 }