fix: run phpunit locally

[hirasso]

Fixes #22

Rename phpunit.dist.xml to phpunit.xml so that phpunit can find it. Using sane defaults will help others quickly running tests, even without explicitly using composer test

Drive-By Fixes

• Make sure to always run the project-specific version of phpunit
• Remove prohibited attributes in phpunit.xml:

- Element 'phpunit', attribute 'strict': The attribute 'strict' is not allowed.
- Element 'phpunit', attribute 'syntaxCheck': The attribute 'syntaxCheck' is not allowed.

[asika32764]

This is not consider as a bug, everyone can just copy phpunit.dist.xml to phpunit.xml to run test. the phpunit.xml in .gitignore is to let developers configure their test files, so it ignores the main phpunit.xml file.

I will close this PR. However it is still welcome if someone wants to create PR to add the test instruction.