Use proper prefix inside router_interface test

[rileykarson]

This left a couple subnetworks floating around (which ends up leaving networks and ultimately routes)

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 2 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 1 file changed, 2 insertions(+), 2 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=136484"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccActiveDirectoryDomain_activeDirectoryDomainBasicExample|TestAccActiveDirectoryDomain_update|TestAccComputeRouterInterface_withTunnel|TestAccComputeRouterInterface_basic|TestAccEndpointsService_basic|TestAccFolderIamAuditConfig_multiple You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=136485"