docs: add README for tests

axi/tb/axi_cocotb_loopback_test/README.md

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+# AXI compact interface cocotb loopback test
+
+Тест интерфейсов и конвертеров flat-if.
+
+```text
+cocotb AxiMaster
+  -> flat AXI signals
+  -> axi4_flat_to_if
+  -> compact axi4_if
+  -> axi4_loopback DUT
+  -> compact axi4_if
+  -> axi4_if_to_flat
+  -> flat AXI signals
+  -> cocotb AxiRam
+```
+
+Зависимости
+
+```bash
+pip install cocotb cocotbext-axi
+```