magefile: added combined test coverage support for `mage test:all` #6737

Job	Run time
paths-filter	5s
Integration Tests	0s
Datastore Integration Tests (crdb)	4s
Datastore Consistency Tests (crdb)	5s
Build Binary & Image	0s
Datastore Integration Tests (mysql)	4s
Datastore Consistency Tests (mysql)	6s
Datastore Integration Tests (postgres, 13.8)	4s
Datastore Integration Tests (spanner)	4s
Datastore Consistency Tests (spanner)	5s
Datastore Integration Tests (postgres, 14)	4s
Datastore Integration Tests (postgres, 15)	7s
Steelthread	0s
Unit	0s
Datastore Integration Tests (postgres, 16)	4s
Datastore Integration Tests (postgres, 17)	4s
E2E	0s
Datastore Integration Tests (pgbouncer, 13.8)	5s
Datastore Consistency Tests (postgres, 13.8)	3s
Datastore Integration Tests (pgbouncer, 14)	4s
Datastore Consistency Tests (postgres, 14)	3s
Analyzers Unit Tests	0s
Datastore Integration Tests (pgbouncer, 15)	4s
Datastore Consistency Tests (postgres, 15)	3s
Datastore Integration Tests (pgbouncer, 16)	5s
Datastore Consistency Tests (postgres, 16)	2s
Datastore Integration Tests (pgbouncer, 17)	5s
Datastore Consistency Tests (postgres, 17)	6s
WASM Tests	0s
Generate Protobufs	0s
	1m 36s

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