Consolidate utility testing functions where possible

[evan-forbes]

As we've discussed synchronously many times, there's quite a bit of redundant testing utility functions. We should consolidate where possible. If its not possible due to cyclic imports, then we should consider putting those tests in their own package.

related to #829

[amoghrajesh]

@evan-forbes I can work on this issue. Can you point me to the files to get started?

[rootulp]

A candidate sub-task for this issue is to delete the duplicate definitions of randomValidNamespace:

1. celestia-app/testutil/payment/testutil.go

   Line 134 in 27632dd

   func randomValidNamespace() namespace.ID {

2. celestia-app/app/test_util.go

   Line 178 in e18de88

   func randomValidNamespace() namespace.ID {

3. celestia-app/app/test/block_size_test.go

   Line 299 in 8f4fd81

   func randomValidNamespace() namespace.ID {

4. celestia-app/pkg/prove/proof_test.go

   Line 290 in 906fdf7

   func randomValidNamespace() namespace.ID {

5. celestia-app/testutil/utils.go

   Line 15 in 07b5bb4

   func RandomValidNamespace() namespace.ID {

The last function definition is the most preferred because it accounts for the parity and tail padding namespaces. Note: the duplicate functions are defined in different packages so the last function may need to be extracted to a package that can be imported by all usages of the duplicate definitions.

[amoghrajesh]

@rootulp @evan-forbes can we define a scope for this issue? We can not achieve all the changes in one PR. If we can scope them, it would be easier to work on

[rootulp]

Spun out #836 in-case @evan-forbes has other ideas for what this issue should include.

[evan-forbes]

another instance per this #1991 (comment)

[rootulp]

Closing this b/c it's not actionable. Please re-open or create a new issue with more specific examples of the redundant testing utilities that need consolidation.