docs(modem): Added description of manual test procedure

components/esp_modem/test/README.md

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+# ESP-Modem Testing
+
+This folder contains automated and manual tests for esp-modem component. Beside these tests, some jobs are executed in CI to exercise standard examples (please refer to the CI definition and CI related sdkconfigs in examples).
+
+List of test projects:
+
+* `host_test` -- esp_modem is build on host (linux), modem's terminal in mocked using Loobpack class which creates simple responders to AT and CMUX mode. This test is executed in CI.
+* `target`  -- test executed on target with no modem device, just a pppd running on the test runner. This test is executed in CI.
+* `target_ota` -- Manual test which perform OTA over PPP.
+* `target_iperf` -- Manual test to measure data throughput via PPP.
+
+## Manual testing
+
+Prior to every `esp_modem` release, these manual tests must be execute and pass:
+
+### `target_ota`
+
+Make sure that the UART ISR is not in IRAM, so the error messages are expected in the log, but the ESP32 should recover and continue with downloading the image.
+
+Perform the test for these devices
+* SIM7600 (CMUX mode)
+* BG96 (CMUX mode)
+* SIM7000 (PPP mode)
+* A7672 (CMUX mode -- the only device with 2 byte CMUX payload), so the test is expected to fail more often if (`CONFIG_ESP_MODEM_CMUX_DEFRAGMENT_PAYLOAD=y` && `CONFIG_ESP_MODEM_USE_INFLATABLE_BUFFER_IF_NEEDED=n` && dte_buffer < device max payload)
+* NetworkDCE -- no modem device, pppd (PPP mode)
+
+Perform the test with these configurations:
+* CONFIG_TEST_USE_VFS_TERM (y/n)
+* CONFIG_ESP_HTTP_CLIENT_ENABLE_CUSTOM_TRANSPORT (y/n)
+* CONFIG_ESP_MODEM_CMUX_DEFRAGMENT_PAYLOAD (y/n)
+* CONFIG_ESP_MODEM_USE_INFLATABLE_BUFFER_IF_NEEDED (y/n)
+
+**Criteria for passing the test**
+
+The test should complete the download with maximum 1 retry (50% of OTA failure)
+In case of CMUX mode, we're trying to exit CMUX at the end of the test. This step might also fail for some devices, as the CMUX-exit is not supported on certain devices (when the final error message appears that the device failed to exit CMUX, we just verify the new image by reseting the ESP32)
+
+### `target_iperf`
+
+Run these 4 `iperf` configurations (either manually or using `pytest`):
+* tcp_tx_throughput
+* tcp_rx_throughput
+* udp_tx_throughput
+* udp_rx_throughput
+
+And verify in all four cases the value is about 0.70 Mbps