Update test suite to phpunit 9.5

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.

tests/DnsMasqTest.php

@@ -10,17 +10,16 @@
 use function Valet\swap;
 use Illuminate\Container\Container;
 
-class DnsMasqTest extends PHPUnit_Framework_TestCase
+class DnsMasqTest extends Yoast\PHPUnitPolyfills\TestCases\TestCase
 {
-    public function setUp()
+    public function set_up()
     {
         $_SERVER['SUDO_USER'] = user();
 
         Container::setInstance(new Container);
     }
 
-
-    public function tearDown()
+    public function tear_down()
     {
         exec('rm -rf '.__DIR__.'/output');
         mkdir(__DIR__.'/output');
@@ -29,7 +28,6 @@ public function tearDown()
         Mockery::close();
     }
 
-
     public function test_install_installs_and_places_configuration_files_in_proper_locations()
     {
         $brew = Mockery::mock(Brew::class);
@@ -56,7 +54,6 @@ public function test_install_installs_and_places_configuration_files_in_proper_l
         );
     }
 
-
     public function test_update_tld_removes_old_resolver_and_reinstalls()
     {
         $cli = Mockery::mock(CommandLine::class);
@@ -68,7 +65,6 @@ public function test_update_tld_removes_old_resolver_and_reinstalls()
     }
 }
 
-
 class StubForCreatingCustomDnsMasqConfigFiles extends DnsMasq
 {
     public function dnsmasqUserConfigDir()