TST: align longtable test with the recent changes

Test for longtable with shorcaption was changed
to reflect the recent changes on master branch.
The changes in longtable environment were introduced recently,
see GH pandas-dev#34360.

Author: ivanovmg

pandas/tests/io/formats/test_to_latex.py

@@ -700,6 +700,11 @@ def test_to_latex_longtable_caption_label(self):
 \toprule
 {} &  a &   b \\
 \midrule
+\endfirsthead
+\caption[]{a table in a \texttt{longtable} environment} \\
+\toprule
+{} &  a &   b \\
+\midrule
 \endhead
 \midrule
 \multicolumn{3}{r}{{Continued on next page}} \\