fix hasChar() for CMAP fonts and add tests (fix #330)

[Connum]

Description

For fonts using CMAP, hasChar() would always return true, even if the char was not contained. This wasn't caught because there was no corresponding test.

Motivation and Context

Fixes #330

How Has This Been Tested?

Added a new test loading the existing CMAP test font and checking the hasChar() return values for existing and non-existing chars.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the README accordingly.
• I have read the CONTRIBUTING document.