Restore wxConvAuto in wxTextInputStream used by wxExecute()

This reverts commit a05ae051d8 and allows
to correctly decode UTF-8 output of child processes again.

Also add a (disabled by default) test allowing to verify that this does
work now.

Closes #14720, #18382.

tests/exec/exec.cpp

@@ -516,3 +516,28 @@ void ExecTestCase::TestOverlappedSyncExecute()
     CPPUNIT_ASSERT_EQUAL( SLEEP_END_STRING, longSleepOutput.Last() );
 #endif // !__WINDOWS__
 }
+
+#ifdef __UNIX__
+
+// This test is disabled by default because it must be run in French locale,
+// i.e. with explicit LC_ALL=fr_FR.UTF-8 and only works with GNU ls, which
+// produces the expected output.
+TEST_CASE("wxExecute::RedirectUTF8", "[exec][unicode][.]")
+{
+    wxArrayString output;
+    REQUIRE( wxExecute("/bin/ls --version", output) == 0 );
+
+    for ( size_t n = 0; n < output.size(); ++n )
+    {
+        // It seems unlikely that this part of the output will change for GNU
+        // ls, so check for its presence as a sign that the program output was
+        // decoded correctly.
+        if ( output[n].find(wxString::FromUTF8("vous \xc3\xaates libre")) != wxString::npos )
+            return;
+    }
+
+    INFO("output was:\n" << wxJoin(output, '\n'));
+    FAIL("Expected output fragment not found.");
+}
+
+#endif // __UNIX__