Reenable integrations tests

As part of https://github.com/dotnet/format/pull/1276/checks?check_run_id=3428592004, some integration tests for some repos were disabled because they failed to build.

failing builds https://dev.azure.com/dnceng/public/_build/results?buildId=1319567&view=logs&j=b55a745c-1d7b-57ac-3901-a041660d6877

msbuild & project-system:
```console
08/26/2021 02:48:44 - Finding solutions.
08/26/2021 02:48:45 - ProjectSystem.sln - Formatting Workspace
Unhandled exception. System.IO.FileLoadException: Could not load file or assembly 'System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The located assembly's manifest definition does not match the assembly reference. (0x80131040)
File name: 'System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

08/26/2021 02:48:49 - Formatting failed with error code -532462766.
##[error]Cmd.exe exited with code '1'.
Finishing: Run dotnet-format on dotnet/project-system ProjectSystem.sln
```

ef6:
```console
08/26/2021 02:53:28 - Running Build.ps1 -restore
Determining projects to restore...
All projects are up-to-date for restore.
dotnet-install: Note that the intended use of this script is for Continuous Integration (CI) scenarios, where:
dotnet-install: - The SDK needs to be installed without user interaction and without admin rights.
dotnet-install: - The SDK installation doesn't need to persist across multiple CI runs.
dotnet-install: To set up a development environment or to run apps, use installers rather than this script. Visit https://dotnet.microsoft.com/download to get the installer.

dotnet-install: Downloading primary link https://dotnetcli.azureedge.net/dotnet/Runtime/3.1.0-rtm.19565.2/dotnet-runtime-3.1.0-rtm.19565.2-win-x64.zip
dotnet-install: Extracting zip from https://dotnetcli.azureedge.net/dotnet/Runtime/3.1.0-rtm.19565.2/dotnet-runtime-3.1.0-rtm.19565.2-win-x64.zip
dotnet-install: Adding to current process PATH: "D:\a\1\s\temp\ef6\.dotnet\". Note: This change will not be visible if PowerShell was run as a child process.
dotnet-install: Note that the script does not resolve dependencies during installation.
dotnet-install: To check the list of dependencies, go to https://docs.microsoft.com/dotnet/core/install/windows#dependencies
dotnet-install: Installation finished
Determining projects to restore...
Could not execute because the specified command or file was not found.
Restored D:\a\1\s\temp\ef6\test\FunctionalTests.ProviderAgnostic\FunctionalTests.ProviderAgnostic.csproj (in 9.87 sec).
Possible reasons for this include:
13 of 14 projects are up-to-date for restore.
* You misspelled a built-in dotnet command.

* You intended to execute a .NET program, but dotnet-D:\a\1\s/artifacts/bin/dotnet-format/Release/net6.0/dotnet-format.dll does not exist.
Build succeeded.
* You intended to run a global tool, but a dotnet-prefixed executable with this name could not be found on the PATH.
0 Warning(s)
0 Error(s)

Time Elapsed 00:00:16.31
08/26/2021 02:53:45 - Finding solutions.
08/26/2021 02:53:45 - EntityFramework.sln - Formatting Workspace

08/26/2021 02:53:46 - Formatting failed with error code 1.
##[error]Cmd.exe exited with code '1'.
Finishing: Run dotnet-format on dotnet/ef6 EntityFramework.sln
```