Revert "router/testing: rename Instance to FakeRouter"

This reverts commit b1ca652671ee6c6fbcd91e4b606658503f115117.
Revert "router/testing: unexport FakeRouter"

This reverts commit 0d9cc24ca1dc38fa399d7f90d25d46f9ed985101.
Revert "router/testing: make the registered router public"

This reverts commit 91d8af0072a8195322985641490638a13f54e452.
Revert "router/testing: improve implementation"

This reverts commit a0ec206ad3139079c2ac442d06fdf2761608c008.