add memory_filesystem test

dev/tests/memory_filesystem.lua

@@ -0,0 +1,53 @@
+app.create_memory_device("memtest")
+
+debug.log("check initial state")
+assert(file.exists("memtest:"))
+assert(file.is_writeable("memtest:"))
+
+debug.log("write text file")
+assert(file.write("memtest:text.txt", "example, пример"))
+assert(file.exists("memtest:text.txt"))
+
+debug.log("read text file")
+assert(file.read("memtest:text.txt") == "example, пример")
+
+debug.log("delete file")
+file.remove("memtest:text.txt")
+assert(not file.exists("memtest:text.txt"))
+
+debug.log("create directory")
+file.mkdir("memtest:dir")
+assert(file.isdir("memtest:dir"))
+
+debug.log("remove directory")
+file.remove("memtest:dir")
+assert(not file.isdir("memtest:dir"))
+
+debug.log("create directories")
+file.mkdirs("memtest:dir/subdir/other")
+assert(file.isdir("memtest:dir/subdir/other"))
+
+debug.log("remove tree")
+file.remove_tree("memtest:dir")
+assert(not file.isdir("memtest:dir"))
+
+debug.log("write binary file")
+local bytes = {0xDE, 0xAD, 0xC0, 0xDE}
+file.write_bytes("memtest:binary", bytes)
+assert(file.exists("memtest:binary"))
+
+debug.log("write binary file")
+local bytes = {0xDE, 0xAD, 0xC0, 0xDE}
+file.write_bytes("memtest:binary", bytes)
+assert(file.exists("memtest:binary"))
+
+debug.log("read binary file")
+local rbytes = file.read_bytes("memtest:binary")
+assert(#rbytes == #bytes)
+for i, b in ipairs(bytes) do
+    assert(rbytes[i] == b)
+end
+
+debug.log("delete file")
+file.remove("memtest:binary")
+assert(not file.exists("memtest:binary"))