fix(test): Fix flaky previous session finalization test

Use awaitility to wait for the previous session file to be deleted
instead of asserting immediately. The PreviousSessionFinalizer runs
as a separate task after the test's task in the single-threaded
executor, so there is a race between the assertion and the file
deletion.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: adinauer

sentry/src/test/java/io/sentry/SentryTest.kt

@@ -964,7 +964,9 @@ class SentryTest {
     }
 
     await.untilTrue(triggered)
-    assertFalse(previousSessionFile.exists())
+    // The PreviousSessionFinalizer runs as a separate task after the test's task in the
+    // single-threaded executor, so we need to wait for it to delete the file too.
+    await.until { !previousSessionFile.exists() }
   }
 
   @Test