Assorted cleanups to reduce trust / notary imports

[thaJeztah]

• relates to cli/command: remove NotaryClient from CLI interface #5876

cli/trust: make NotaryServer a const

This var used to be vendored from github.com/docker/docker/registry, but was
removed there, and made a local var in a1cbaa8.

It is (and should never be) modified, so let's change it into a const.

internal/test/notary: add testPassRetriever helper

Add a basic helper to provide the equivalent of passphrase.ConstantRetriever
with a fixed passphrase for testing.

cli/command/trust: remove unused passphrase-retriever from test

The test only validates that an error is produced because the notary
server is offline, and does not sent a passphrase.

cli/command/trust: remove TestGetOrGenerateNotaryKeyAndInitRepo

This test was only testing trust.GetSignableRoles to return an error
if it's offline, which was duplicating the TestGetSignableRolesError
test.

cli/command/trust: remove TestGetSignableRolesForTargetAndRemoveError

This test was only testing trust.GetSignableRoles to return an error
if it's offline, which was duplicating the TestGetSignableRolesError
test.

cli/command/trust: add testPassRetriever helper

Add a basic helper to provide the equivalent of passphrase.ConstantRetriever
with a fixed passphrase for testing.

cli/command/image: remove TestAddTargetToAllSignableRolesError

This test was only testing trust.GetSignableRoles to return an error
if it's offline, which was duplicating the TestGetSignableRolesError
test in the cli/trust package.

cli/command/image: use t.SetEnv in trust tests

cli/command/image: move trust unit-tests to trust package

These tests were not testing functionality that was implemented
in the image package. Move them to the trust package, where
they belong.

cli/command/trust: use gotest.tools in tests

cli/command/image: move AddTargetToAllSignableRoles to cli/trust

This utility was shared between the "image" and "trust" packages, and a
shallow wrapper around features in the cli/trust package. Move it there
instead and rename it to trust.AddToAllSignableRoles.

There are no known external consumers of this utility, so skipping a
deprecation.

cli/command/image: rename vars that shadowed type

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 10.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 59.28%. Comparing base (fe0a8d2) to head (c7072a8).
Report is 17 commits behind head on master.

❌ Your patch status has failed because the patch coverage (10.00%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5878      +/-   ##
==========================================
- Coverage   59.32%   59.28%   -0.04%     
==========================================
  Files         353      353              
  Lines       29748    29752       +4     
==========================================
- Hits        17647    17639       -8     
- Misses      11113    11127      +14     
+ Partials      988      986       -2