Exclude tests/**/TestFixtures/**/*.csproj from Arcade build enumeration

The Stabilization Check job invokes Arcade's pack pipeline, which
enumerates ProjectToBuild from `tests/**/*.csproj` (eng/Build.props
line 69). That enumeration was picking up the static-analyzer fixture
projects under tests/Infrastructure.Tests/TestSelector/TestFixtures/.
Those .csproj files are deliberate test fixtures consumed as text by
TestSelector's analyzer tests (IgnorePathFilterTests,
NuGetDependentTestDetectorTests, etc.) — they're never meant to be
built. Their adjacent Directory.Build.targets neuters Build/Rebuild/
Restore/Pack/Test as no-ops, which is enough for `build.sh -build`
but not for `build.sh -pack`: the NuGet Pack target still attempts
to read the assembly output (e.g. ConditionalPackableProject.dll) and
fails with NU5026 because Build was a no-op.

Filter the fixtures out of the test-project enumeration so Arcade
never tries to build or pack them. The TestSelector tool's own
testProjectPatterns config already excludes `tests/**/TestFixtures/**`
from its discovery, so this brings the build infrastructure in line
with the same convention.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: radical

eng/Build.props

@@ -66,7 +66,8 @@
         included in the build at all. This is useful for cases like when we are
         just building the packages, and don't need to build the test projects. -->
   <ItemGroup Condition="'$(SkipManagedBuild)' != 'true' and '$(SkipTestProjects)' != 'true' and '$(BuildBundleDepsOnly)' != 'true'">
-    <ProjectToBuild Include="$(RepoRoot)tests\**\*.csproj" />
+    <ProjectToBuild Include="$(RepoRoot)tests\**\*.csproj"
+                    Exclude="$(RepoRoot)tests\**\TestFixtures\**\*.csproj" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(SkipNativeBuild)' == 'true'">