add docs for integration tests

docs/porting/porting_full_process/integration tests.md

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+# Integration tests
+
+Mbed OS provides a set of integration tests to help you identify integration issues around storage devices and connectivity devices. If you're using a custom board, these tests can also help you verify your board works well in your environment.
+
+## Test suites
+
+| **Test suite** | **Description** |
+| ------------- | ------------- |
+| `fs-single` | File system single-threaded tests with write buffer sizes - 1 byte, 4b, 16b, 64b, 256b, 1kb, 4kb, 16kb, 64kb. |
+| `fs-multi` | File system multithreaded test with write buffer sizes - 1 byte, 4b, 16b, 64b, 256b, 1kb, 4kb, 16kb, 64kb. |
+| `net-single` | Network single-threaded test with receive buffer sizes - 128 bytes, 256b, 1kb, 2kb, 4kb. |
+| `net-multi` | Network multithreaded test for 1, 2 and 3 download threads with 1kb receive buffer size. |
+| `stress-net-fs` | Network and file system single and multithreaded tests:<ul><li>memory allocation - 10k, 20k, 40k, 60k</li><li>1 thread (sequential) - 1 download (1kb buffer), 1 file thread (1kb buffer)</li><li>2 parallel threads - 1 download, 1 file thread (1kb buffer)</li><li>3 parallel threads - 1 download, 2 file (256 bytes, 1 kb buffer)</li><li>4 parallel threads - 1 download, 3 file (1 byte, 256 bytes, 1kb buffer)</li></ul> |
+
+## Execute tests
+
+The integration test requires you to configure both the connectivity and storage devices.
+
+If a platform has both of them enabled by default (in `targets.json`), then you can compile the tests and run them from inside the `mbed-os` directory: 
+
+    ```
+    $ mbed test -t <toolchain> -m <platform> -n *integration-* -DINTEGRATION_TESTS -v
+    ```
+
+If a board doesn't have default connectivity and storage, it requires an extra shield to run the integration test. In this case, you can pass a `target_extended.json` file in the command-line to configure the target:
+
+   ```
+   $ mbed test -t <toolchain> -m <platform> -n *integration-* -DINTEGRATION_TESTS --app-config TESTS\integration\COMMON\target_extended.json -v
+   ```
+
+A preconfiguration file is defined in the COMMON folder, and that file covers some of the platform connectivity and storage combinations. However, this preconfiguration file might contain some compoments drivers not included in `mbed-os` (for example, `WIFI_WIZFI310` and `NUSD`). You must add those drivers manually. If you have your own platform or want to define your own configuration, please follow the [configuration guide](#tests-configuration).
+
+## Tests configuration
+
+If required, edit the `target_extended.json` file, and create a new entry under `target_overrides` with the target name for your device:
+
+- **Connectivity** - Specify the default connectivity type for your target. It's essential with targets that lack default connectivity set in `targets.json` or for targets that support multiple connectivity options. For example:
+
+   ```
+            "target.network-default-interface-type" : "ETHERNET",
+   ```
+
+   The possible options are `ETHERNET`, `WIFI` and `CELLULAR`.
+
+   Depending on connectivity type, you might have to specify more configuration options. For an example, please see the defined `CELLULAR` targets in `mbed_app.json`.
+
+- **Storage** - Specify the storage block device type, which dynamically adds the block device driver you specified at compile time. For example:
+
+   ```
+            "target.components_add" : ["SD"],
+   ```
+
+   Valid options are `SD`, `SPIF` and `QSPIF`. For more available options, please see the [block device components](https://github.com/ARMmbed/mbed-os/tree/master/components/storage/blockdevice).
+
+   You also have to specify the block device pin configuration, which may vary from one block device type to another. Here's an example for `SD`:
+
+   ```
+            "sd.SPI_MOSI"                      : "PE_6",
+            "sd.SPI_MISO"                      : "PE_5",
+            "sd.SPI_CLK"                       : "PE_2",
+            "sd.SPI_CS"                        : "PE_4",
+   ```
+
+   If you are using SPI or QSPI flash, please make sure you have specified the correct SPI frequency by configuring `spif-driver.SPI_FREQ`. If it is not configured, 40Mhz is applied by default.
+