Add 3 tests

[szepeviktor]

$ composer test

> find src/ -type f -name '*.php' -print0|xargs -0 -L 1 -- php -l
No syntax errors detected in src/Dismiss.php
No syntax errors detected in src/Notice.php
No syntax errors detected in src/Notices.php
> phpcs --standard=WordPress-Core --exclude=Generic.Arrays.DisallowShortArraySyntax src/
> phpstan analyze
Note: Using configuration file /home/viktor/src/admin-notices/phpstan.neon.dist.
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 [OK] No errors

@aristath

[szepeviktor]

You're welcome.