Add some fstring tests.

test/test_fstring.py

@@ -0,0 +1,24 @@
+import pytest
+
+from parso import load_grammar
+
+
+@pytest.fixture
+def grammar():
+    return load_grammar(language="python-f-string")
+
+
+@pytest.mark.parametrize(
+    'code', [
+        '{1}',
+        '',
+        '{1!a}',
+        '{1!a:1}',
+        '{1:1}',
+        '{1:1.{32}}',
+    ]
+)
+def test_valid(code, grammar):
+    fstring = grammar.parse(code, error_recovery=False)
+    assert fstring.type == 'fstring'
+    assert fstring.get_code() == code