builtin: use C.fwrite (buffered) for _write_buf_to_fd (#14558)

cmd/tools/modules/scripting/scripting.v

@@ -24,6 +24,7 @@ pub fn cprint(omessage string) {
 		message = term.cyan(message)
 	}
 	print(message)
+	flush_stdout()
 }
 
 pub fn cprint_strong(omessage string) {
@@ -32,16 +33,19 @@ pub fn cprint_strong(omessage string) {
 		message = term.bright_green(message)
 	}
 	print(message)
+	flush_stdout()
 }
 
 pub fn cprintln(omessage string) {
 	cprint(omessage)
 	println('')
+	flush_stdout()
 }
 
 pub fn cprintln_strong(omessage string) {
 	cprint_strong(omessage)
 	println('')
+	flush_stdout()
 }
 
 pub fn verbose_trace(label string, message string) {

cmd/tools/modules/testing/common.v

@@ -123,6 +123,7 @@ pub fn (mut ts TestSession) print_messages() {
 			// progress mode, the last line is rewritten many times:
 			if is_ok && !ts.silent_mode {
 				print('\r$empty\r$msg')
+				flush_stdout()
 			} else {
 				// the last \n is needed, so SKIP/FAIL messages
 				// will not get overwritten by the OK ones