backup: remove TODOTestTenantDisabled for tests

[shubhamdhama]

This change is part of our effort to eliminate the remaining usages of
TODOTestTenantDisabled from tests across various features. It removes
its occurrence in the backup tests.

Informs: #138912

Epic: CRDB-38970

Release note: None

[cockroach-teamcity]

This change is 

[shubhamdhama]

There was zero to almost no effort in enabling tenant testing for most of them, but not that straightforward to do that for TestExportRequestBelowGCThresholdOnDataExcludedFromBackup and TestExcludeDataFromBackupDoesNotHoldupGC. They fail because backpressure doesn't build up in upsertUntilBackpressure. Steven and I took a high level look and maybe the reason it doesn't build-up here is secondary tenant's spans are not included in backpressure checks:

cockroach/pkg/kv/kvserver/replica_backpressure.go

Lines 86 to 94 in e048c1b

	// backpressurableSpans contains spans of keys where write backpressuring
	// is permitted. Writes to any keys within these spans may cause a batch
	// to be backpressured.
	var backpressurableSpans = []roachpb.Span{
	{Key: keys.TimeseriesPrefix, EndKey: keys.TimeseriesKeyMax},
	// Backpressure from the end of the system config forward instead of
	// over all table data to avoid backpressuring unsplittable ranges.
	{Key: keys.SystemConfigTableDataMax, EndKey: keys.TableDataMax},
	}