Fix intermittent test failures

[l0b0]

Some of the tests assume that QGIS will finish some asynchronous processing by the time the result is needed in the test code, leading to test failures:

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ERROR: test_should_filter_wfs_by_map_bbox (test_ldi_integration.UserWorkFlows)
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Traceback (most recent call last):
  File "/tests_directory/linz-data-importer/tests/test_ldi_integration.py", line 321, in test_should_filter_wfs_by_map_bbox
    self.filter_data_by_map("NZ Primary Parcels")
  File "/tests_directory/linz-data-importer/tests/test_ldi_integration.py", line 474, in filter_data_by_map
    layer = QgsProject.instance().mapLayersByName(layer_name)[0]
IndexError: list index out of range

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ERROR: test_importDataset_wfs (test_ldi_plugin.UnitLevel)
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Traceback (most recent call last):
  File "/tests_directory/linz-data-importer/tests/test_ldi_plugin.py", line 484, in test_importDataset_wfs
    self.assertEqual(self.ldi.layer_title, names[0])
IndexError: list index out of range

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FAIL: test_wfs_import (test_ldi_integration.UserWorkFlows)
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Traceback (most recent call last):
  File "/tests_directory/linz-data-importer/tests/test_ldi_integration.py", line 293, in test_wfs_import
    self.import_service("wfs")
  File "/tests_directory/linz-data-importer/tests/test_ldi_integration.py", line 421, in import_service
    self.assertEqual(len(names), nconfs)
AssertionError: 0 != 1

Instead of relying on the timing of QGIS the tests should poll the state or subscribe to an event stream which eventually results in the given change.