Correct fish_wcstod and fix Linux build failure

Limit the fish_wcstod fast path to ASCII digits only, to fix the problem
observed in the discussion for a700acadfa
where LANG=de_DE.UTF-8 would cause `test` to interpret commas instead of
periods inside floating point values.

src/fish_tests.cpp

@@ -6,6 +6,7 @@
 #include <errno.h>
 #include <libgen.h>
 #include <limits.h>
+#include <math.h>
 #include <pthread.h>
 #include <signal.h>
 #include <stdarg.h>