Fix typo and don't use 'a' as an invalid filename since it is the default

gcc binary filename and is thus not unlikely to be left lying around in the
build dir which cause these tests to fail

ext/standard/tests/strings/sha1_file.phpt

@@ -38,7 +38,7 @@ echo "\n-- No filename --\n";
 var_dump( sha1_file("") );
 
 echo "\n-- invalid filename --\n";
-var_dump( sha1_file("a") );
+var_dump( sha1_file("rewncwYcn89q") );
 
 echo "\n-- Scalar value as filename --\n";
 var_dump( sha1_file(12) );
@@ -82,7 +82,7 @@ bool(false)
 
 -- invalid filename --
 
-Warning: sha1_file(a): failed to open stream: No such file or directory in %s on line %d
+Warning: sha1_file(rewncwYcn89q): failed to open stream: No such file or directory in %s on line %d
 bool(false)
 
 -- Scalar value as filename --