Add tests for superscript plugin

Fixtures file is copied from the `markdown-it-sup` tests:
https://github.com/markdown-it/markdown-it-sup/blob/master/test/fixtures/sup.txt

Test is modelled on other Markdown-It-Py inline tests.

Author: elijahgreenstein

tests/fixtures/superscript.md

@@ -0,0 +1,48 @@
+.
+^test^
+.
+<p><sup>test</sup></p>
+.
+
+.
+^foo\^
+.
+<p>^foo^</p>
+.
+
+.
+2^4 + 3^5
+.
+<p>2^4 + 3^5</p>
+.
+
+.
+^foo~bar^baz^bar~foo^
+.
+<p><sup>foo~bar</sup>baz<sup>bar~foo</sup></p>
+.
+
+.
+^\ foo\ ^
+.
+<p><sup> foo </sup></p>
+.
+
+.
+^foo\\\\\\\ bar^
+.
+<p><sup>foo\\\ bar</sup></p>
+.
+
+.
+^foo\\\\\\ bar^
+.
+<p>^foo\\\ bar^</p>
+.
+
+.
+**^foo^ bar**
+.
+<p><strong><sup>foo</sup> bar</strong></p>
+.
+

tests/test_superscript.py

@@ -0,0 +1,23 @@
+from pathlib import Path
+
+from markdown_it import MarkdownIt
+from markdown_it.utils import read_fixture_file
+import pytest
+
+from mdit_py_plugins.superscript import superscript_plugin
+
+FIXTURE_PATH = Path(__file__).parent.joinpath("fixtures")
+
+
+@pytest.mark.parametrize(
+    "line,title,input,expected",
+    read_fixture_file(FIXTURE_PATH.joinpath("superscript.md")),
+)
+def test_superscript_fixtures(line, title, input, expected):
+    md = MarkdownIt("commonmark").use(superscript_plugin)
+    if "DISABLE-CODEBLOCKS" in title:
+        md.disable("code")
+    md.options["xhtmlOut"] = False
+    text = md.render(input)
+    print(text)
+    assert text.rstrip() == expected.rstrip()