io: add a test for read_all (#6898)

vlib/io/reader_test.v

@@ -0,0 +1,40 @@
+module io
+
+struct Buf {
+pub:
+	bytes []byte
+mut:
+	i     int
+}
+
+fn (mut b Buf) read(mut buf []byte) ?int {
+	if !(b.i < b.bytes.len) {
+		return eof
+	}
+	n := copy(buf, b.bytes[b.i..b.bytes.len])
+	b.i += n
+	return n
+}
+
+fn test_read_all() {
+	buf := Buf{
+		bytes: '123'.repeat(10).bytes()
+	}
+	res := read_all(buf) or {
+		assert false
+		''.bytes()
+	}
+	assert res == '123'.repeat(10).bytes()
+}
+
+/*
+TODO: This test failed by a bug of read_all
+fn test_read_all_huge() {
+	buf := Buf{bytes: "123".repeat(100000).bytes()}
+	res := read_all(buf) or {
+		assert false
+		"".bytes()
+	}
+	assert res == "123".repeat(100000).bytes()
+}
+*/