katex-test: Add widget tests for \overline and \underline

Author: MdSaifAliMolla

lib/widgets/katex.dart

@@ -128,7 +128,7 @@ class _KatexSpan extends StatelessWidget {
       final borderColor = color ?? DefaultTextStyle.of(context).style.color!;
       final borderWidth = styles.borderBottomWidthEm! * em;
 
-      widget = DecoratedBox(
+      widget = Container(
         decoration: BoxDecoration(
           border: Border(
             bottom: BorderSide(color: borderColor, width: borderWidth, style: BorderStyle.solid))),

test/widgets/katex_test.dart

@@ -81,6 +81,14 @@ void main() {
         ('a', Offset(2.47, 3.36), Size(10.88, 25.00)),
         ('b', Offset(15.81, 3.36), Size(8.82, 25.00)),
       ]),
+      (KatexExample.overline, skip: false, [
+        ('A', Offset(0.0, 5.61), Size(15.43, 25.0)),
+        ('B', Offset(15.43, 5.61), Size(15.61, 25.0)),
+      ]),
+      (KatexExample.underline, skip: false, [
+        ('A', Offset(0.0, 5.61), Size(15.43, 25.0)),
+        ('B', Offset(15.43, 5.61), Size(15.61, 25.0)),
+      ]),
     ];
 
     for (final testCase in testCases) {