Reduce parallelism for composite crossgen2 for jit64_3 test

src/tests/Common/CLRTest.CrossGen.targets

@@ -32,6 +32,10 @@ WARNING:   When setting properties based on their current state (for example:
     Tests can opt out of ahead-of-time (AOT) compilation from crossgen / crossgen2 by setting this property in their project:
 
       <CrossGenTest>false</CrossGenTest>
+
+    Tests can opt out of composite crossgen2 only by setting this property in their project:
+
+      <CompositeCrossGenTest>false</CompositeCrossGenTest>
   -->
   <Target Name="GetCrossgenBashScript">
 
@@ -46,6 +50,11 @@ if [ "$(AlwaysUseCrossGen2)" == "true" ]; then
     export CompositeBuildMode=1
 fi
 
+# CompositeBuildMode is an opt-in by variable presence.
+if [ "$(CompositeCrossGenTest)" = "false" ] && [ -n "${CompositeBuildMode+x}" ]; then
+    unset RunCrossGen2
+fi
+
 # CrossGen2 Script
 if [ ! -z ${RunCrossGen2+x} ]%3B then
   compilationDoneFlagFile="IL-CG2/done"
@@ -232,6 +241,13 @@ if /i "$(AlwaysUseCrossGen2)" == "true" (
     set CompositeBuildMode=1
 )
 
+if /i "$(CompositeCrossGenTest)" == "false" (
+    REM CompositeBuildMode is an opt-in by variable presence.
+    if defined CompositeBuildMode (
+        set RunCrossGen2=
+    )
+)
+
 REM CrossGen2 Script
 if defined RunCrossGen2 (
     set ExtraCrossGen2Args=!ExtraCrossGen2Args! $(CrossGen2TestExtraArguments)

src/tests/JIT/jit64/jit64_3.csproj

@@ -1,4 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <CompositeCrossGenTest>false</CompositeCrossGenTest>
+  </PropertyGroup>
+
   <ItemGroup>
     <MergedWrapperProjectReference Include="opt/**/*.??proj" />
   </ItemGroup>