magefile: added combined test coverage support for `mage test:all` #6774

Job	Run time
paths-filter	9s
Steelthread	0s
Integration Tests	0s
Build Binary & Image	0s
Unit	0s
Analyzers Unit Tests	0s
Datastore Consistency Tests (crdb)	5s
Datastore Integration Tests (crdb)	5s
WASM Tests	0s
Datastore Consistency Tests (mysql)	4s
Datastore Integration Tests (mysql)	4s
E2E	0s
Datastore Consistency Tests (spanner)	4s
Datastore Integration Tests (spanner)	5s
Datastore Consistency Tests (postgres, 13.8)	5s
Datastore Consistency Tests (postgres, 14)	5s
Datastore Consistency Tests (postgres, 15)	5s
Datastore Integration Tests (postgres, 13.8)	4s
Datastore Consistency Tests (postgres, 16)	4s
Generate Protobufs	0s
Datastore Integration Tests (postgres, 14)	5s
Datastore Consistency Tests (postgres, 17)	5s
Datastore Integration Tests (postgres, 15)	4s
Datastore Integration Tests (postgres, 16)	5s
Datastore Integration Tests (postgres, 17)	5s
Datastore Integration Tests (pgbouncer, 13.8)	5s
Datastore Integration Tests (pgbouncer, 14)	4s
Datastore Integration Tests (pgbouncer, 15)	4s
Datastore Integration Tests (pgbouncer, 16)	4s
Datastore Integration Tests (pgbouncer, 17)	4s
	1m 44s

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