[Testing (DUP)] Add /internal/testutil pkg & refactor mockdns test code

[bryanchriswhite]

Description

Factors out some common mockdns usage, including a "noop" logger to things down (mockdns is noisy IMO).

(Duplicate of #685 🙄, I forgot to change the base branch before merging. As a result, the changes were squashed-merged into a different branch than main. Re-opening here, apologies for the extra noise. 🙏🙏)

Issue

Fixes #609

Type of change

Please mark the relevant option(s):

• New feature, functionality or library
• Bug fix
• Code health or cleanup
• Major breaking change
• Documentation
• Other

List of changes

• Create /internal directory
• Add /interal/testutil pkg
• Refactor existing mockdns usage

Testing

• Ensure test util package members are not present in symbol table of non-test builds:
  1. go build -o ./pokt-node ./app/pocket
  2. nm ./pokt-node | grep prepareDNSResolverMock should print nothing
     If prepareDNSResolverMock was included in the build, it will be listed in the symbol table which we can print with nm. Double-check the function names match, a typo could result in a false positive. Additionally, one can build a test binary (go test -c -o <output bin path> [-run <test func name>] <pkg>) on a test pkg which is known to import a test util function and ensure that it is present in the symbol table, as a control.

• make develop_test; if any code changes were made
• Docker Compose LocalNet; if any major functionality was changed or introduced
• k8s LocalNet; if any infrastructure or configuration changes were made

Required Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added, or updated, godoc format comments on touched members (see: tip.golang.org/doc/comment)
• I have tested my changes using the available tooling
• I have updated the corresponding CHANGELOG

If Applicable Checklist

• I have updated the corresponding README(s); local and/or global
• I have added tests that prove my fix is effective or that my feature works
• I have added, or updated, mermaid.js diagrams in the corresponding README(s)
• I have added, or updated, documentation and mermaid.js diagrams in shared/docs/* if I updated shared/*README(s)