Move to a process global for test MemoryTracker (#4739)

I've seen about three or four different test failures due to
`create_test_memory_tracker` returning new instances which leads to life
time issues with the memory resources. This changes to a single process
singleton to avoid the issue altogether.

Once we merge the current PRs that are all using
`create_test_memory_tracker` I'll follow up with a PR to rename all of
those instances to `get_test_memory_tracker` and remove the
`create_test_memory_tracker` to prevent further use.

---
TYPE: NO_HISTORY
DESC: Move to a process global for test MemoryTracker

test/support/src/mem_helpers.cc

@@ -35,15 +35,25 @@
 
 namespace tiledb::test {
 
-shared_ptr<sm::MemoryTracker> create_test_memory_tracker() {
+shared_ptr<sm::MemoryTracker> get_test_memory_tracker() {
   class MemoryTrackerCreator : public sm::MemoryTracker {
    public:
     MemoryTrackerCreator()
         : sm::MemoryTracker() {
     }
+
+    static shared_ptr<MemoryTracker> get_instance() {
+      static shared_ptr<MemoryTrackerCreator> tracker{
+          new MemoryTrackerCreator()};
+      return tracker;
+    }
   };
 
-  return make_shared<MemoryTrackerCreator>(HERE());
+  return MemoryTrackerCreator::get_instance();
+}
+
+shared_ptr<sm::MemoryTracker> create_test_memory_tracker() {
+  return get_test_memory_tracker();
 }
 
 }  // namespace tiledb::test

test/support/src/mem_helpers.h

@@ -38,6 +38,18 @@
 
 namespace tiledb::test {
 
+/**
+ * Helper function get the test instance of a shared_ptr<MemoryTracker>
+ *
+ * This is the preferred function. The create_test_memory_tracker will be
+ * replaced shortly and only serves as a proxy to this function while we
+ * transition the first few PRs to use this new function.
+ *
+ * The reasoning here is that creating memory trackers has turned out to be a
+ * bit of a footgun with lifetime issues.
+ */
+shared_ptr<sm::MemoryTracker> get_test_memory_tracker();
+
 /**
  * Helper function to create test instances of shared_ptr<MemoryTracker>
  */