lib/errors: errnotest use os.exit not raw syscall (rule-7)

The local doexit reimplemented os.exit via a raw rt_syscall(60) decl
with no documented divergence, while the file already imports + uses
os. os.exit (lib/os/os.ww:68) is byte-identical (syscall1(nr.EXIT=60));
ostest/stattest siblings already use os.exit.

lib/errors/errnotest.ww

@@ -11,15 +11,10 @@ package errors;
 import errors;
 import os;
 
-@symbol("rt_syscall") fn syscall1ww(num: i64, a: i64) i64;
-fn doexit(code: i32) void = {
-	syscall1ww(60i64, code: i64);
-};
-
 // signalled — bumped by main before each test so a failing exit code
 // pinpoints the offending case.
 let signalled: i32 = 0;
-fn fail() void = { doexit(signalled + 10); };
+fn fail() void = { os.exit(signalled + 10); };
 
 fn streq(a: str, b: str) bool = {
 	if (a.len != b.len) { return false; };