Make test error when number is invalid nicer

This would print the default "Argument is invalid" error string, which
is *true* but not super obvious, because `test` doesn't always perform
numeric conversion, and that's the bit that failed here.

src/builtin_test.cpp

@@ -716,6 +716,8 @@ static bool parse_number(const wcstring &arg, number_t *number, wcstring_list_t
             errors.push_back(_(L"Not a number"));
         } else if (std::isinf(floating)) {
             errors.push_back(_(L"Number is infinite"));
+        } else if (errno == EINVAL) {
+            errors.push_back(format_string(L"Argument is not a number: '%ls'", argcs));
         } else {
             errors.push_back(format_string(L"%s: '%ls'", std::strerror(errno), argcs));
         }

tests/checks/test.fish

@@ -37,7 +37,7 @@ function t
 end
 
 t foo
-# CHECKERR: Invalid argument: 'foo'
+# CHECKERR: Argument is not a number: 'foo'
 # CHECKERR: {{.*}}test.fish (line {{\d+}}):
 # CHECKERR: test $argv[1] -eq 5
 # CHECKERR: ^