Add read access checking

[arksap2002]

Description of changes made

Fixing an error:

com.intellij.openapi.diagnostic.RuntimeExceptionWithAttachments: Read access is allowed from inside read-action or Event Dispatch Thread (EDT) only (see Application.runReadAction()); see https://jb.gg/ij-platform-threading for details
Current thread: Thread[AWT-AppKit,5,system] 125169378 (EventQueue.isDispatchThread()=false)
SystemEventQueueThread: Thread[AWT-EventQueue-0,6,main] 302538171
	at com.intellij.openapi.application.impl.ApplicationImpl.createThreadAccessException(ApplicationImpl.java:1083)
	at com.intellij.openapi.application.impl.ApplicationImpl.assertReadAccessAllowed(ApplicationImpl.java:1038)
	at com.intellij.psi.impl.PsiManagerImpl.findFile(PsiManagerImpl.java:155)
	at org.jetbrains.research.testspark.services.TestCaseDisplayService$applyTests$1.invoke(TestCaseDisplayService.kt:313)
	at org.jetbrains.research.testspark.services.TestCaseDisplayService$applyTests$1.invoke(TestCaseDisplayService.kt:310)
	at org.jetbrains.research.testspark.services.TestCaseDisplayService.applyTests$lambda$9(TestCaseDisplayService.kt:310)
	at com.intellij.openapi.fileChooser.FileChooserDescriptor.isFileSelectable(FileChooserDescriptor.java:260)
	at com.intellij.ui.mac.MacPathChooserDialog.lambda$choose$2(MacPathChooserDialog.java:96)
	at com.intellij.openapi.fileChooser.FileChooser.lambda$safeInvokeFilter$1(FileChooser.java:143)
	at java.desktop/sun.lwawt.macosx.CFileDialog.queryFilenameFilter(CFileDialog.java:188)

Other notes

Closes ICTL-793

• I have checked that I am merging into correct branch