testing/nettest: Add README.md under nettest directory.

testing/nettest/README.md

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+# Net Test Module
+
+The `apps/testing/nettest` directory is used to build testcases related
+to the network protocol stack. You can use it on any device with network
+protocol stack capabilities.
+
+## Directory Structure
+
+```
+├──tcp
+   ├── test_tcp_connect_ipv4.c  # The testcase of TCP connect
+   ├── ...
+   ├── test_tcp_common.c        # TCP testcase common function
+   ├── test_tcp.h               # TCP testcases declaration
+   └── test_tcp.c               # TCP testcase execution entry
+├──udp
+   ├── ...
+   ├── test_udp.h               # UDP testcases declaration
+   └── test_udp.c               # UDP testcase execution entry
+├── ...
+├──utils                        # Utils for testcases
+   ├── ...
+   ├── nettest_netdump.c
+   ├── nettest_tcpserver.c
+   └── utils.h
+├── CmakeLists.txt
+├── Kconfig                     # Enable module testcases
+├── Make.defs
+└── Makefile
+```
+
+## How to Build
+
+Firstly, all testcases in this directory rely on the cmocka framework,
+so please ensure that the following cmocka configuration is turned on.
+
+- `CONFIG_ALLOW_MIT_COMPONENTS=y`
+- `CONFIG_LIBC_REGEX=y`
+- `CONFIG_TESTING_CMOCKA=y`
+
+Then, open the network part testcase configuration.
+
+- `CONFIG_TESTING_NET_TEST=y`
+
+Finally, you can choose which protocol testcase to enable through
+configuration, some protocol configurations depend on other network
+configurations, you can view the dependencies in the `Kconfig` file. Take
+the TCP testcase as an example, the configuration is as follows:
+
+- `CONFIG_NET_TCPBACKLOG=y`  - TCP testcase compilation dependencies
+- `CONFIG_NET_TCP=y`         - TCP testcase compilation dependencies
+- `CONFIG_TESTING_NET_TCP=y` - TCP testcase configuration items, for other
+                               protocol, just replace the tailing protocol
+                               name
+
+## How to Run
+
+After the testcase is built, you can run the testcase at command line through
+the following command.
+
+- `cmocoka --test test_tcp_connect_ipv4` - Run a single testcase
+
+- `cmocka_net_tcp` - Run TCP testcase(for other protocol, just replace
+                                      the tailing protocol name)
+
+In addition when you run `cmocka`, the above command will also be included.
+
+## How to Add Testcases
+
+If you want to add testcases, please follow the following steps:
+
+- Add the testcase source file to the corresponding directory. Please follow the
+following rules for naming source files.
+`test_<protocol>_<function description>_<additional description>.c`
+such as `test_tcp_connect_ipv4.c`.
+Test case names are the same as source files, such as `test_tcp_connect_ipv4()`.
+
+- Add the testcase source file to `CMakeLists.txt` file.
+
+- Add the testcase source file to `Makefile` file.
+
+- Add the testcase to the corresponding declaration file and execute entry file. such as
+`test_tcp_connect_ipv4()` needs to be added to `test_tcp.h` and `test_tcp.c`. If you
+have a separate setup function, also add it to the above two files.