Update test suite to phpunit 9.5 syntax
Refactored to use polyfill for older PHP versions via `yoast/phpunit-polyfills`
Note: this includes 2 important differences from usual phpunit test suites:
- instead of extending `PHPUnit\Framework\TestCase` we extend `Yoast\PHPUnitPolyfills\TestCases\TestCase`
- instead of handling fixtures via `setUp()` and `tearDown()` we use `set_up()` and `tear_down()` respectively
Comment regarding formatting: I chose to use the FQDN in the `extends` syntax of the class declaration instead of using `use` so that it is more quickly apparent that we're doing something slightly different than usual phpunit syntax, particularly in regards to the set_up() / tear_down() methods that appear immediately following the `extends` line.
Fixes#144
".test" or ".dev" is really a TLD, not a "domain" in the conventional sense.
Changing the command to `valet tld` more accurately reflects the purpose of the command (to set or get the configured TLD served by Valet)
The use of `valet domain` is currently preserved as an alias for `valet tld`, but will be removed at a later date.
