Add regression test for multiline setext headings (#19155)

Co-authored-by: cyphercodes <cyphercodes@users.noreply.github.com>

tests/format/markdown/heading/setext/__snapshots__/format.test.js.snap

@@ -107,6 +107,23 @@ Heading
 ================================================================================
 `;
 
+exports[`issue-19148.md format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+Foo
+Bar
+---
+
+=====================================output=====================================
+Foo
+Bar
+---
+
+================================================================================
+`;
+
 exports[`setext.md format 1`] = `
 ====================================options=====================================
 parsers: ["markdown"]

tests/format/markdown/heading/setext/issue-19148.md

@@ -0,0 +1,3 @@
+Foo
+Bar
+---