[Test] Reset ApplicationManager.application when shared application environments are disposed

- Shared application environments can be disposed in the middle of test
  plan execution when the project count reaches zero. So resetting
  `ApplicationManager` only on "test plan execution finished" is
  incorrect, because it does not account for that scenario. Instead, we
  should reset `ApplicationManager` every time the application
  environment is disposed.
- We should also keep the manual reset in `testPlanExecutionFinished`
  because `disposeApplicationEnvironment` may not need to dispose
  anything if there isn't a shared application environment, but we
  should still reset `ApplicationManager` if an unshared application
  environment was created.
- This fixes "Some test disposed, but forgot to clear MockApplication"
  exceptions which appeared after KT-64099 because it opened up proper
  disposal of the shared application environment (instead of it being in
  a state of leakage because not all project disposables were properly
  disposed).

^KT-63650
^KT-64099

compiler/cli/cli-base/src/org/jetbrains/kotlin/cli/jvm/compiler/KotlinCoreEnvironment.kt

@@ -16,6 +16,7 @@ import com.intellij.ide.highlighter.JavaFileType
 import com.intellij.lang.java.JavaParserDefinition
 import com.intellij.mock.MockProject
 import com.intellij.openapi.Disposable
+import com.intellij.openapi.application.Application
 import com.intellij.openapi.application.ApplicationManager
 import com.intellij.openapi.application.TransactionGuard
 import com.intellij.openapi.application.TransactionGuardImpl
@@ -594,10 +595,37 @@ class KotlinCoreEnvironment private constructor(
                 val environment = ourApplicationEnvironment ?: return
                 ourApplicationEnvironment = null
                 Disposer.dispose(environment.parentDisposable)
+                resetApplicationManager(environment.application)
                 ZipHandler.clearFileAccessorCache()
             }
         }
 
+        /**
+         * Resets the application managed by [ApplicationManager] to `null`. If [applicationToReset] is specified, [resetApplicationManager]
+         * will only reset the application if it's the expected one. Otherwise, the application will already have been changed to another
+         * application. For example, application disposal can trigger one of the disposables registered via
+         * [ApplicationManager.setApplication], which reset the managed application to the previous application.
+         */
+        @JvmStatic
+        fun resetApplicationManager(applicationToReset: Application? = null) {
+            val currentApplication = ApplicationManager.getApplication() ?: return
+            if (applicationToReset != null && applicationToReset != currentApplication) {
+                return
+            }
+
+            try {
+                val ourApplicationField = ApplicationManager::class.java.getDeclaredField("ourApplication")
+                ourApplicationField.isAccessible = true
+                ourApplicationField.set(null, null)
+            } catch (exception: Exception) {
+                // Resetting the application manager is not critical in a production context. If the reflective access fails, we shouldn't
+                // expose the user to the failure.
+                if (currentApplication.isUnitTestMode) {
+                    throw exception
+                }
+            }
+        }
+
         @JvmStatic
         fun ProjectEnvironment.configureProjectEnvironment(
             configuration: CompilerConfiguration,