docs: add testing framework document (#142)

Author: nghuyenthevinh2000

tests/README.md

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+# I. Why we need to setup an automated testing framework?
+In many cases, we have observed that unit test isn't enough to guarantee that the whole system will not break. We simply got lucky when Ed found a problem through his manual system testing that may not always happen.
+
+In some cases, not even the unit test is correct due to writer misunderstanding of required logic.
+
+# II. Framework
+1. Standarize writing unit test (we can even go further with a ci to check). A PR should include:
+* feature specification (in written markdown file in a folder called feature-request) (Gherkin language)
+* unit test write
+
+2. a full - fledged e2e test flow through ci that does following flow
+* upgrade testing:
+    * for chain (if exists, else use current terrad): do an upgrade gov test to newer chain binary
+    * for wasm (if there are changes to wasm, else use current wasm files): change to newer wasm files
+* regression testing (change to system should not break existing one)
+    * backend: 
+        * chain should run normal 
+        * ibc should work normal
+        * wasm should run normal
+        * state sync should run normal
+    * frontend: api testing of most important transaction types and query
+* fuzz testing:
+    * some nice background reading: https://github.com/osmosis-labs/osmosis/blob/main/simulation/ADR.md
+    * we need to care only about events that invoke a state change, we then create randomized request to these events:
+        * transaction: create randomized request and broadcast multiple times
+        * begin block: randomized environment condition that invokes a begin block logic (Ex: system time, number of validators, ...)
+        * end block: randomized environment condition that invokes an end block logic (Ex: system time, number of validators, ...)
+        * init genesis: create randomized genesis state
+
+It is hard to invoke a begin, end block logic without first understanding what environment condition it relies on.