minor WBT fixups for local testing (#87817)

Now that there's no perftrace variant, the _BuildVariants item group
could be empty.  Also replace "three" by "two" and 3 by 2 in a few places

Author: lambdageek

eng/testing/tests.browser.targets

@@ -303,8 +303,8 @@
       <_RuntimePackNugetAvailable Remove="@(_RuntimePackNugetAvailable)" Condition="$([System.String]::new('%(_RuntimePackNugetAvailable.FileName)').EndsWith('.symbols'))" />
     </ItemGroup>
 
-    <Error Condition="@(_RuntimePackNugetAvailable -> Count()) != 3 and @(_RuntimePackNugetAvailable -> Count()) != 1"
-           Text="Expected to find either one or three in $(LibrariesShippingPackagesDir): @(_RuntimePackNugetAvailable->'%(FileName)%(Extension)')" />
+    <Error Condition="@(_RuntimePackNugetAvailable -> Count()) != 2 and @(_RuntimePackNugetAvailable -> Count()) != 1"
+           Text="Expected to find either one or two in $(LibrariesShippingPackagesDir): @(_RuntimePackNugetAvailable->'%(FileName)%(Extension)')" />
 
     <ItemGroup>
       <_BuildVariants Include="multithread" Condition="'$(_DefaultBuildVariant)' != '.multithread.'" />
@@ -313,7 +313,8 @@
                       Project="$(InstallerProjectRoot)pkg/sfx/Microsoft.NETCore.App/Microsoft.NETCore.App.Runtime.sfxproj"
                       Dependencies="$(_DefaultRuntimePackNuGetPath)"
                       Properties="@(_DefaultPropsForNuGetBuild, ';');MonoWasmBuildVariant=%(_BuildVariants.Identity)"
-                      Descriptor="runtime pack for %(_BuildVariants.Identity)" />
+                      Descriptor="runtime pack for %(_BuildVariants.Identity)"
+                      Condition="'%(_BuildVariants.Identity)' != ''"/>
 
       <!-- add for non-threaded runtime also -->
       <_NuGetsToBuild Include="$(LibrariesShippingPackagesDir)Microsoft.NETCore.App.Runtime.Mono.$(RuntimeIdentifier).$(PackageVersionForWorkloadManifests).nupkg"
@@ -330,7 +331,7 @@
         Text="
       ********************
 
-      Note: Could not find the expected three runtime packs in $(LibrariesShippingPackagesDir). Found @(_RuntimePackNugetAvailable->'%(FileName)%(Extension)', ', ') .
+      Note: Could not find the expected two runtime packs in $(LibrariesShippingPackagesDir). Found @(_RuntimePackNugetAvailable->'%(FileName)%(Extension)', ', ') .
             To support local builds, the same runtime pack will be built with the other variant names.
             To disable this behavior, pass `-p:WasmSkipMissingRuntimePackBuild=true` .