Fix culture-dependent test assertion for FormatTimeWithExplicitFormatString

Changed assertion to use regex pattern that matches both 12-hour and 24-hour time formats.
The test was failing in en-US culture where "T" format produces "1:02:03 AM" instead of "01:02:03".

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/libraries/System.Private.Xml/tests/Xslt/XslCompiledTransformApi/XslCompilerTests.cs

@@ -181,7 +181,8 @@ public void FormatTimeWithExplicitFormatString()
                 }
 
                 string result = outWriter.ToString();
-                Assert.Contains("01:02:03", result);
+                Assert.DoesNotContain("2001", result);
+                Assert.Matches(@"0?1:02:03", result);
             }
         }
     }