Fleaky test ResponseFileSwitchesAppearInCommandLine

### Context

Failing CI test on Mac

```
Shouldly.ShouldAssertException : \"MSBUILD : error MSB1060: Undefined environment variable passed in as switch.\r\n Full command line:...\"\n should contain (case insensitive comparison)\n\": %NONEXISTENTENVIRONMENTVARIABLE%\"\n but did not
```

Full output of the test

```
MSBUILD : error MSB1060: Undefined environment variable passed in as switch.
Full command line: '/Users/runner/work/1/s/artifacts/bin/Microsoft.Build.CommandLine.UnitTests/Debug/net7.0/MSBuild.dll project.proj -nologo'
Switches appended by response files:
'' came from '/Users/runner/work/1/s/artifacts/bin/Microsoft.Build.CommandLine.UnitTests/Debug/net7.0/MSBuild.rsp'
'' came from '/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/jol2i3fv.gsq/Directory.Build.rsp'
'-nowarn:MSB1001 @myRsp.rsp %NONEXISTENTENVIRONMENTVARIABLE%' came from '/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/jol2i3fv.gsq/MSBuildTemprunner/Temporary02d0d4329a7440e5a9b6306760054ce9/MSBuild.rsp'
'-nr:false -m:2' came from '/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/jol2i3fv.gsq/MSBuildTemprunner/Temporary02d0d4329a7440e5a9b6306760054ce9/myRsp.rsp'
```

Unexpectedly missing:

```
Switch: %NONEXISTENTENVIRONMENTVARIABLE%

For switch syntax, type "MSBuild -help"

```

so it looks like `CommandLineSwitchException` was not thrown

Start by locating and running the ResponseFileSwitchesAppearInCommandLine test, focusing on the Mac CI output and response-file command-line parsing. Done means the test reliably throws CommandLineSwitchException and its assertion includes %NONEXISTENTENVIRONMENTVARIABLE% on Mac.

Written by the indexing model from the issue text.