Revert usage of System.Collections.Immutable for TypeCache

We have received some complaints from some users running MSTest on old machines where this package is not available (no .NET installed) causing some runtime load error.

src/Adapter/MSTest.TestAdapter/Execution/TypeCache.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Globalization;
 using System.Linq;
 using System.Reflection;
@@ -72,24 +71,24 @@ internal TypeCache(ReflectHelper reflectionHelper)
     /// <summary>
     /// Gets Class Info cache which has cleanup methods to execute.
     /// </summary>
-    public ImmutableArray<TestClassInfo> ClassInfoListWithExecutableCleanupMethods =>
-        _classInfoCache.Values.Where(classInfo => classInfo.HasExecutableCleanupMethod).ToImmutableArray();
+    public IReadOnlyCollection<TestClassInfo> ClassInfoListWithExecutableCleanupMethods =>
+        _classInfoCache.Values.Where(classInfo => classInfo.HasExecutableCleanupMethod).ToList();
 
     /// <summary>
     /// Gets Assembly Info cache which has cleanup methods to execute.
     /// </summary>
-    public ImmutableArray<TestAssemblyInfo> AssemblyInfoListWithExecutableCleanupMethods =>
-        _testAssemblyInfoCache.Values.Where(assemblyInfo => assemblyInfo.HasExecutableCleanupMethod).ToImmutableArray();
+    public IReadOnlyCollection<TestAssemblyInfo> AssemblyInfoListWithExecutableCleanupMethods =>
+        _testAssemblyInfoCache.Values.Where(assemblyInfo => assemblyInfo.HasExecutableCleanupMethod).ToList();
 
     /// <summary>
     /// Gets the set of cached assembly info values.
     /// </summary>
-    public ImmutableArray<TestAssemblyInfo> AssemblyInfoCache => _testAssemblyInfoCache.Values.ToImmutableArray();
+    public IReadOnlyCollection<TestAssemblyInfo> AssemblyInfoCache => _testAssemblyInfoCache.Values.ToList();
 
     /// <summary>
     /// Gets the set of cached class info values.
     /// </summary>
-    public ImmutableArray<TestClassInfo> ClassInfoCache => _classInfoCache.Values.ToImmutableArray();
+    public IReadOnlyCollection<TestClassInfo> ClassInfoCache => _classInfoCache.Values.ToList();
 
     /// <summary>
     /// Get the test method info corresponding to the parameter test Element.