Adjust throw-in-read-in-a-mode test

The test is not[^1] part of 6.c-errata.

We don't actually bother to read if we reached EOF, which is the case
with empty file the test uses, so no throwage would occur just Nil/empty buf
would be returned (which I'm personally fine with).

Adjust test to use .slurp for reading which still does throw in :a open mode.

[1] 091931a717aa0507d04fcef10b81

Author: zoffixznet

S32-io/open.t

@@ -204,7 +204,7 @@ LEAVE unlink PATH;
     ok defined($fh), 'can open existing file in :a mode';
     $fh.print('cats say meow');
 
-    throws-like { $fh.get }, Exception, 'trying to read in :a mode throws';
+    throws-like { $fh.slurp }, Exception, 'trying to read in :a mode throws';
     $fh.close;
 
     $fh = open PATH, :r;