Rename 'testable' with 'testables'

[devxoul]

Rename testable with testables as @wplong11 suggested.

This PR breaks existing API:

- profileViewController.testable[\.usernameLabel]
+ profileViewController.testables[\.usernameLabel]

[codecov]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #2   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          13     13           
=====================================
  Hits           13     13

Impacted Files	Coverage Δ
Sources/Testables/Testables.swift	100% <100%> (ø)

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[wplong11]

🙏