mem: add test for Write/Seek

Signed-off-by: Yerden Zhumabekov <yerden.zhumabekov@gmail.com>

mem/file_test.go

@@ -1,6 +1,7 @@
 package mem
 
 import (
+	"bytes"
 	"io"
 	"testing"
 	"time"
@@ -205,3 +206,42 @@ func TestFileReadAtSeekOffset(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestFileWriteAndSeek(t *testing.T) {
+	fd := CreateFile("foo")
+	f := NewFileHandle(fd)
+
+	assert := func(expected bool, v ...interface{}) {
+		if !expected {
+			t.Helper()
+			t.Fatal(v...)
+		}
+	}
+
+	data4 := []byte{0, 1, 2, 3}
+	data20 := bytes.Repeat(data4, 5)
+	var off int64
+
+	for i := 0; i < 100; i++ {
+		// write 20 bytes
+		n, err := f.Write(data20)
+		assert(err == nil, err)
+		off += int64(n)
+		assert(n == len(data20), n)
+		assert(off == int64((i+1)*len(data20)), off)
+
+		// rewind to start and write 4 bytes there
+		cur, err := f.Seek(-off, io.SeekCurrent)
+		assert(err == nil, err)
+		assert(cur == 0, cur)
+
+		n, err = f.Write(data4)
+		assert(err == nil, err)
+		assert(n == len(data4), n)
+
+		// back at the end
+		cur, err = f.Seek(off-int64(n), io.SeekCurrent)
+		assert(err == nil, err)
+		assert(cur == off, cur, off)
+	}
+}