fix and chore: refactor tests to minimize initializing DB connections

SUMMARY
    The test suite was spinning up way too many database connections that what's
    strictly needed and so the test suite was failing with[0]:
	code: "53300", message: "sorry, too many clients already"

EXPERIMENTS
    Tried sharing database connection pool across all tests with
    async_once[0] but faced:
	- IO errors
	    The connections were probably getting dropped in between tests
	- actix Actor errors
	    The actor was probably not getting initialized before a
	    a reference to the async_once initialized app
	    context(crate::data::Data) is retrieved and used

FIX
    crate::tests was spinning up an App context
    instance(crate::data::Data) for most utility functions, which was
    unnecessarily excessive.

    Each test now creates an instance of the application context at the
    beginning and shared a reference with all test utility functions. So
    number of database connections/app context instance = number of unit
    tests.

[0]: permanently fixes #22
[1]: https://docs.rs/async_once/latest/async_once/

Makefile

@@ -97,8 +97,8 @@ test: frontend-test frontend ## Run all available tests
 	cd db/db-sqlx-postgres &&\
 		DATABASE_URL=${POSTGRES_DATABASE_URL}\
 		cargo test --no-fail-fast
-	./scripts/tests.sh
-#	cargo test --all-features --no-fail-fast
+	cargo test --all-features --no-fail-fast
+#	./scripts/tests.sh
 
 xml-test-coverage: migrate ## Generate code coverage report in XML format
 	cargo tarpaulin -t 1200 --out Xml