More tests on cell_markers=""" (#856)

tests/test_cell_markers.py

@@ -0,0 +1,33 @@
+from nbformat.v4.nbbase import new_raw_cell
+
+from jupytext import reads, writes
+from jupytext.cli import jupytext
+
+
+def test_set_cell_markers_cli(tmpdir, cwd_tmpdir):
+    tmpdir.join("test.py").write("# %% [markdown]\n# A Markdown cell\n")
+    jupytext(["--format-options", 'cell_markers="""', "test.py"])
+    py = tmpdir.join("test.py").read()
+    assert py.endswith('# %% [markdown]\n"""\nA Markdown cell\n"""\n')
+
+
+def test_add_cell_to_script_with_cell_markers(
+    no_jupytext_version_number,
+    py='''# ---
+# jupyter:
+#   jupytext:
+#     formats: py:percent
+#     cell_markers: '"""'
+# ---
+''',
+):
+    nb = reads(py, fmt="py:percent")
+    nb.cells = [new_raw_cell("A raw cell")]
+    py2 = writes(nb, fmt="py:percent")
+    assert py2.endswith(
+        '''# %% [raw]
+"""
+A raw cell
+"""
+'''
+    )