Test consolidation

[nibra]

Reorganisation and completion of tests

This package still uses phputf8, which has long been abandoned.
In preparation of an exchange (see #29), this PR increases the test coverage from ~75% to >96%.
A few small issues have necessarily been solved on the way.

Summary of Changes

Tests

• Add test cases
• Improve testdox annotations
• Add type hints

StringHelper

• Reduce complexity to increase path/branch coverage

Inflector

• Remove internal use of deprecated code

phputf8

• Fix bug in str_ireplace() - count variable was not populated

Testing Instructions

• Code Review. Make sure that signatures in StringHelper and Inflector did not change in an incompatible way. Changes to signatures should only affect default values.

Documentation Changes Required

None.