Add test for text fontsize in SVG circuit diagrams (#8161)

Address mutation check in cl/935515593

Author: pavoljuhas

cirq-core/cirq/contrib/svg/svg_test.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import re
+
 import IPython.display
 import numpy as np
 import pytest
@@ -24,9 +26,16 @@ def test_svg() -> None:
             cirq.MatrixGate(np.eye(2)).on(a),
         )
     )
-    assert '?' in svg_text
     assert '<svg' in svg_text
     assert '</svg>' in svg_text
+    # check text rendering fontsize
+    # single letter X gate rendered at 18px
+    assert re.search(r'<text[^>]*\bfont-size="18px[^>]*>X</text>', svg_text)
+    # multi-letter PhasedXPowGate rendered at 14px
+    phx_literal = re.escape("PhX(0.456)^0.123")
+    assert re.search(rf'<text[^>]*\bfont-size="14px[^>]*>{phx_literal}</text>', svg_text)
+    # MatrixGate replaced with "?" rendered at 18px
+    assert re.search(r'<text[^>]*\bfont-size="18px[^>]*>[?]</text>', svg_text)
 
 
 def test_svg_noise() -> None: