fix: rename test directories to match SPM testTarget naming convention

Rename mParticle-BrazeTest → mParticle-BrazeTests (braze-12/13/14) and
mParticle-FirebaseTest → mParticle-FirebaseTests (firebase-11/12) so
directory names match the testTarget names declared in each Package.swift.

swift-tools-version 5.5 requires an exact match; 5.9 allowed mismatches
as warnings. This resolves the CI package-resolution failures introduced
by the swift-tools-version downgrade.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nickolas-dimitrakas