[ty] Run file watching tests serial when using nextest (#20918)

.config/nextest.toml

@@ -1,3 +1,12 @@
+# Define serial test group for running tests sequentially.
+[test-groups]
+serial = { max-threads = 1 }
+
+# Run ty file watching tests sequentially to avoid race conditions.
+[[profile.default.overrides]]
+filter = 'binary(file_watching)'
+test-group = 'serial'
+
 [profile.ci]
 # Print out output for failing tests as soon as they fail, and also at the end
 # of the run (for easy scrollability).