docs: add decision record about removing H2

docs/developer/decision-records/2022-08-17-remove_h2_database_tests/README.md

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+# Remove H2 as test database
+
+## Decision
+
+All SQL implementations will be changed so that all their tests target an actual Postgres instance instead of an
+embedded H2. Test code is currently split up in multiple test classes, one targeting H2, and one for Postgres-specific
+code.
+
+## Rationale
+
+While H2 is great for testing generic stuff, it does not support any Postgres-specific features such as JSON queries.
+Also, some datatypes behave slightly differently in Postgres. Thus, other than duplicating all test code, the only
+option left was for developers to decide which tests should target which database, which is error-prone and almost
+impossible to keep consistent. Due to the fact that JUnit `@Tag` annotations are inherited, we cannot simply let the
+Postgres tests extend the H2 test, because then excluding them from a test run would not work anymore.
+
+> For that reason the decision was made to only target Postgres, completely ditching H2.
+
+## Approach
+
+- There will be an `abstract` test class, that only contains the `@Test` methods and a reference to the
+  system-under-test. Then there will be a Postgres-specific subclass, that initializes the database and instantiates the
+  system-under-test. For example, there will be an `AssetIndexTest.java` and a `PostgresAssetIndexTest.java`. That way
+  we can easily test multiple SQL implementations against the same test scenario.
+- A JUnit extension will be devised that provides the Postgres database connection to deduplicate code. Currently, it is
+  created in every test separately.
+
+## Future improvements
+
+Once we actually have another SQL implementation next to Postgres, we could split up the modules into a generic SQL one,
+and a postgres specific one, contributing the actual statements and the test code, for example:
+
+```
+extensions
+├── sql
+│   ├── asset-index // <- only contains unit test
+│   ├── contract-definition-store
+│   ├── ...
+├── postgres
+│   ├── asset-index-postgres // <- contributes PostgresDialectStatements.java and PostgresAssetIndexTest.java
+│   ├── contract-definition-store-postgres
+│   ├── ...
+├── mssql 
+│   ├── asset-index-mssql // <- contributes MssqlDialectStatements.java and MssqlAssetIndexTest.java
+│   ├── contract-definition-store-mssql
+│   ├── ...
+
+```