TST: add a test of font families in svg text-as-text mode

This tests:

1. all of the fonts from the generic lists Matplotlib maintains ends up in the
   svg
2. the generic family is included and un-escaped
3. the specific fonts are escaped
4. glyph fallback will happen in fonts found via generic family names

Author: tacaswell

lib/matplotlib/tests/test_backend_svg.py

@@ -527,3 +527,64 @@ def test_svg_escape():
         fig.savefig(fd, format='svg')
         buf = fd.getvalue().decode()
         assert '<'"&>"' in buf
+
+
+@pytest.mark.parametrize("font_str", [
+    "'DejaVu Sans', 'WenQuanYi Zen Hei', 'Arial', sans-serif",
+    "'DejaVu Serif', 'WenQuanYi Zen Hei', 'Times New Roman', serif",
+    "'Arial', 'WenQuanYi Zen Hei', cursive",
+    "'Impact', 'WenQuanYi Zen Hei', fantasy",
+    "'DejaVu Sans Mono', 'WenQuanYi Zen Hei', 'Courier New', monospace",
+    # These do not work because the logic to get the font metrics will not find
+    # WenQuanYi as the fallback logic stops with the first fallback font:
+    # "'DejaVu Sans Mono', 'Courier New', 'WenQuanYi Zen Hei', monospace",
+    # "'DejaVu Sans', 'Arial', 'WenQuanYi Zen Hei', sans-serif",
+    # "'DejaVu Serif', 'Times New Roman', 'WenQuanYi Zen Hei',  serif",
+])
+@pytest.mark.parametrize("include_generic", [True, False])
+def test_svg_font_string(font_str, include_generic):
+    fp = fm.FontProperties(family=["WenQuanYi Zen Hei"])
+    if Path(fm.findfont(fp)).name != "wqy-zenhei.ttc":
+        pytest.skip("Font may be missing")
+
+    explicit, *rest, generic = map(
+        lambda x: x.strip("'"), font_str.split(", ")
+    )
+    size = len(generic)
+    if include_generic:
+        rest = rest + [generic]
+    plt.rcParams[f"font.{generic}"] = rest
+    plt.rcParams["font.size"] = size
+    plt.rcParams["svg.fonttype"] = "none"
+
+    fig, ax = plt.subplots()
+    if generic == "sans-serif":
+        generic_options = ["sans", "sans-serif", "sans serif"]
+    else:
+        generic_options = [generic]
+
+    for generic_name in generic_options:
+        # test that fallback works
+        ax.text(0.5, 0.5, "There are 几个汉字 in between!",
+                family=[explicit, generic_name], ha="center")
+        # test deduplication works
+        ax.text(0.5, 0.1, "There are 几个汉字 in between!",
+                family=[explicit, *rest, generic_name], ha="center")
+    ax.axis("off")
+
+    with BytesIO() as fd:
+        fig.savefig(fd, format="svg")
+        buf = fd.getvalue()
+
+    tree = xml.etree.ElementTree.fromstring(buf)
+    ns = "http://www.w3.org/2000/svg"
+    text_count = 0
+    for text_element in tree.findall(f".//{{{ns}}}text"):
+        text_count += 1
+        font_info = dict(
+            map(lambda x: x.strip(), _.strip().split(":"))
+            for _ in dict(text_element.items())["style"].split(";")
+        )["font"]
+
+        assert font_info == f"{size}px {font_str}"
+    assert text_count == len(ax.texts)