btoc32 tests: Enhance tests

[Bruno Haible via Gnulib discussion list]

With the recent changes to lib/mbrtoc32.c and lib/btoc32.c, it is worth
checking whether the two functions are still in sync. Fortunately, they are.


2026-09-18  Bruno Haible  <br...@clisp.org>

        btoc32 tests: Enhance tests.
        * tests/test-btoc32.c (main): Verify that btoc32 is consistent with
        mbrtoc32.
        * modules/btoc32-tests (Depends-on): Add mbszero, mbrtoc32.

diff --git a/modules/btoc32-tests b/modules/btoc32-tests
index edeccf3f05..182492c0f6 100644
--- a/modules/btoc32-tests
+++ b/modules/btoc32-tests
@@ -11,6 +11,8 @@ m4/codeset.m4
 
 Depends-on:
 setlocale
+mbszero
+mbrtoc32
 streq
 
 configure.ac:
diff --git a/tests/test-btoc32.c b/tests/test-btoc32.c
index a19f0896c5..ae367b7e6e 100644
--- a/tests/test-btoc32.c
+++ b/tests/test-btoc32.c
@@ -38,6 +38,21 @@ main (int argc, char *argv[])
 
   ASSERT (btoc32 (EOF) == WEOF);
 
+  /* Verify that btoc32 is consistent with mbrtoc32.  */
+  for (int c = 0; c < 0x100; c++)
+    if (c != 0)
+      {
+        mbstate_t state;
+        mbszero (&state);
+        char s[1];
+        s[0] = (unsigned char) c;
+        char32_t wc;
+        if (mbrtoc32 (&wc, s, 1, &state) <= 1)
+          ASSERT (btoc32 (c) == wc);
+        else
+          ASSERT (btoc32 (c) == WEOF);
+      }
+
 #ifdef __ANDROID__
   /* On Android ≥ 5.0, the default locale is the "C.UTF-8" locale, not the
      "C" locale.  Furthermore, when you attempt to set the "C" or "POSIX"