magefile: added combined test coverage support for `mage test:all` #6740

Job	Run time
paths-filter	4s
Build Binary & Image	0s
Datastore Consistency Tests (crdb)	5s
Datastore Integration Tests (crdb)	5s
Integration Tests	0s
Datastore Consistency Tests (mysql)	3s
Datastore Integration Tests (postgres, 13.8)	5s
Datastore Integration Tests (mysql)	4s
Datastore Consistency Tests (spanner)	5s
Datastore Integration Tests (postgres, 14)	3s
Datastore Integration Tests (spanner)	4s
Datastore Integration Tests (postgres, 15)	5s
Steelthread	0s
Unit	0s
Datastore Integration Tests (postgres, 16)	3s
Analyzers Unit Tests	0s
Datastore Integration Tests (postgres, 17)	4s
WASM Tests	0s
Datastore Integration Tests (pgbouncer, 13.8)	7s
E2E	0s
Datastore Consistency Tests (postgres, 13.8)	4s
Datastore Integration Tests (pgbouncer, 14)	5s
Datastore Consistency Tests (postgres, 14)	3s
Datastore Integration Tests (pgbouncer, 15)	5s
Generate Protobufs	0s
Datastore Consistency Tests (postgres, 15)	5s
Datastore Integration Tests (pgbouncer, 16)	5s
Datastore Consistency Tests (postgres, 16)	4s
Datastore Integration Tests (pgbouncer, 17)	4s
Datastore Consistency Tests (postgres, 17)	2s
	1m 34s

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