completions.py: Add a test for our artifact ref autocompletions

Since the artifact subcommand group has been created, we have been able
to tab complete both element names and artifact refs as arguments to
the artifact subcommands (e.g. bst artifact log), this commit adds a
test for this.

tests/completions/completions.py

@@ -289,3 +289,45 @@ def test_argument_element_invalid(datafiles, cli, project, cmd, word_idx, expect
 ])
 def test_help_commands(cli, cmd, word_idx, expected):
     assert_completion(cli, cmd, word_idx, expected)
+
+
+@pytest.mark.datafiles(DATA_DIR)
+@pytest.mark.integration
+def test_argument_artifact(datafiles, cli):
+    project = os.path.join(datafiles.dirname, datafiles.basename, 'project')
+    cwd = os.path.join(project)
+
+    # Build an import element with no dependencies (as there will only be ONE cache key)
+    result = cli.run(project=project, args=['build', 'import-bin.bst'])  # Has no dependencies
+    result.assert_success()
+
+    # Get the key and the artifact ref ($project/$element_name/$key)
+    key = cli.get_element_key(project, 'import-bin.bst')
+    artifact = os.path.join('test', 'import-bin', key)
+
+    # Test autocompletion of the artifact
+    cmds = [
+        'bst artifact log ',
+        'bst artifact log t',
+        'bst artifact log test/'
+    ]
+
+    for i, cmd in enumerate(cmds):
+        word_idx = 3
+        result = cli.run(project=project, cwd=cwd, env={
+            '_BST_COMPLETION': 'complete',
+            'COMP_WORDS': cmd,
+            'COMP_CWORD': str(word_idx)
+        })
+        words = []
+        if result.output:
+            words = result.output.splitlines()
+
+            if i == 0:
+                expected = PROJECT_ELEMENTS + [artifact]  # We should now be able to see the artifact
+            elif i == 1:
+                expected = ['target.bst', artifact]
+            elif i == 2:
+                expected = [artifact]
+
+            assert artifact in words