kvserver: add deleted_external_sstable_test tests

This commit adds general tests for the case where we have an external
SSTable pointing to a deleted file. It makes sure that things behave
as expected, and that the server doesn't crash when running different
operations

References: #143135

Release note: None

Author: iskettaneh

pkg/kv/kvserver/BUILD.bazel

@@ -306,6 +306,7 @@ go_test(
         "closed_timestamp_test.go",
         "consistency_queue_test.go",
         "debug_print_test.go",
+        "deleted_external_sstable_test.go",
         "errors_test.go",
         "flow_control_integration_test.go",
         "flow_control_raft_transport_test.go",