update standalone and incremental tests to new API

doc/langref/bad_default_value.zig

@@ -17,7 +17,7 @@ pub fn main() !void {
         .maximum = 0.20,
     };
     const category = threshold.categorize(0.90);
-    try std.io.getStdOut().writeAll(@tagName(category));
+    try std.fs.File.stdout().writeAll(@tagName(category));
 }
 
 const std = @import("std");

doc/langref/hello.zig

@@ -1,8 +1,7 @@
 const std = @import("std");
 
 pub fn main() !void {
-    const stdout = std.io.getStdOut().writer();
-    try stdout.print("Hello, {s}!\n", .{"world"});
+    try std.fs.File.stdout().writeAll("Hello, World!\n");
 }
 
 // exe=succeed

doc/langref/hello_again.zig

@@ -1,7 +1,7 @@
 const std = @import("std");
 
 pub fn main() void {
-    std.debug.print("Hello, world!\n", .{});
+    std.debug.print("Hello, {s}!\n", .{"World"});
 }
 
 // exe=succeed