Add set_file_mtime.test to tests.

Closes #226.

regress/CMakeLists.txt

diff --git a/regress/CMakeLists.txt b/regress/CMakeLists.txt
index 165aeeb..844d9ae 100644
--- a/regress/CMakeLists.txt
+++ b/regress/CMakeLists.txt
@@ -205,6 +205,7 @@
 	set_compression_xz_to_store.test
 	set_compression_zstd_to_store.test
 	set_file_dostime.test
+	set_file_mtime.test
 	stat_index_cp437_guess.test
 	stat_index_cp437_raw.test
 	stat_index_cp437_strict.test