[skip-ci] Add README for ext/mysqli tests

Closes GH-5804

diff --git a/ext/mysqli/tests/README.md b/ext/mysqli/tests/README.md
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+# The mysqli extension tests
+
+To run the tests, a test database must be created in the MySQL command-line:  
+```sql
+CREATE DATABASE test;
+```
+
+The test suite will create the necessary tables for testing, and then delete them when testing is complete. Creating a dedicated table prior to running the tests is unnecessary. There are default values for `MYSQL_TEST_HOST`, `MYSQL_TEST_USER`, `MYSQL_TEST_DB` and `MYSQL_TEST_PASSWD`. If your values differ from the defaults, then environment variables must be set.
+
+```bash
+# Database host
+# Default: localhost
+MYSQL_TEST_HOST
+
+# Database port
+MYSQL_TEST_PORT
+
+# Database user
+# Default: root
+MYSQL_TEST_USER
+
+# Database user password
+# The default password is empty (no password).
+MYSQL_TEST_PASSWD
+
+# Database name
+# Default: test
+MYSQL_TEST_DB
+
+# Storage engine to use
+MYSQL_TEST_ENGINE
+
+# Database server socket
+MYSQL_TEST_SOCKET
+```
+
+## MySQL User Permissions
+
+The MySQL user used to run the tests must have full permissions on the test database.
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