unit test deprecation buffer

tests/unit/test_deprecations.py

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+import pytest
+
+import dbt.deprecations as deprecations
+
+
+@pytest.fixture(scope="function")
+def active_deprecations():
+    assert not deprecations.active_deprecations
+
+    yield deprecations.active_deprecations
+
+    deprecations.reset_deprecations()
+
+
+@pytest.fixture(scope="function")
+def buffered_deprecations():
+    assert not deprecations.buffered_deprecations
+
+    yield deprecations.buffered_deprecations
+
+    deprecations.buffered_deprecations.clear()
+
+
+def test_buffer_deprecation(active_deprecations, buffered_deprecations):
+    deprecations.buffer("project-flags-moved")
+
+    assert active_deprecations == set()
+    assert len(buffered_deprecations) == 1
+
+
+def test_fire_buffered_deprecations(active_deprecations, buffered_deprecations):
+    deprecations.buffer("project-flags-moved")
+    deprecations.fire_buffered_deprecations()
+
+    assert active_deprecations == set(["project-flags-moved"])
+    assert len(buffered_deprecations) == 0