crt: add test for towctrans function

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>

mingw-w64-crt/testcases/Makefile.am

diff --git a/mingw-w64-crt/testcases/Makefile.am b/mingw-w64-crt/testcases/Makefile.am
index 540660f..eb70440 100644
--- a/mingw-w64-crt/testcases/Makefile.am
+++ b/mingw-w64-crt/testcases/Makefile.am
@@ -73,6 +73,7 @@
   t_tmpfile \
   t_trycatch \
   t_stat_slash \
+  t_towctrans \
   t_utime \
   t_vsscanf \
   t_wcrtomb \

mingw-w64-crt/testcases/t_towctrans.c

diff --git a/mingw-w64-crt/testcases/t_towctrans.c b/mingw-w64-crt/testcases/t_towctrans.c
new file mode 100644
index 0000000..978a105
--- /dev/null
+++ b/mingw-w64-crt/testcases/t_towctrans.c
@@ -0,0 +1,46 @@
+/**
+ * This file has no copyright assigned and is placed in the Public Domain.
+ * This file is part of the mingw-w64 runtime package.
+ * No warranty is given; refer to the file DISCLAIMER.PD within this package.
+ */
+
+#include <assert.h>
+#include <locale.h>
+#include <wctype.h>
+
+/**
+ * Test Summary:
+ *
+ * Verify that `towctrans` is consistent with mapping-specific wctype.h
+ * functions.
+ *
+ * `towctrans (wc, wctrans ("tolower"))` must be equivalent to `towlower (wc)`.
+ * `towctrans (wc, wctrans ("toupper"))` must be equivalent to `towupper (wc)`.
+ *
+ * POSIX requires that if second argument to `towctrans` is `(wctrans_t)0`,
+ * then `wc` is returned unchanged.
+ */
+
+int main (void) {
+  if (setlocale (LC_ALL, "English_United States") == NULL) {
+    return 77;
+  }
+
+  wctrans_t lower = 0;
+  wctrans_t upper = 0;
+
+  assert ((lower = wctrans ("tolower")) != (wctrans_t) 0);
+  assert ((upper = wctrans ("toupper")) != (wctrans_t) 0);
+
+  for (wint_t wc = 0;; ++wc) {
+    assert (towctrans (wc, (wctrans_t) 0) == wc);
+    assert (towlower (wc) == towctrans (wc, lower));
+    assert (towupper (wc) == towctrans (wc, upper));
+
+    if (wc == WEOF) {
+      break;
+    }
+  }
+
+  return 0;
+}