[TASK] Use PHPUnit 9.x with PHP 7.3+

This is required to be able to test against PHP 8.  However, PHPUnit 9.x
requires PHP 7.3, so a different version of PHPUnit is required for different
testing environments.

The main change here is a step in the GitHub Action to conditionally update
PHPUnit via PHIVE for PHP >=7.3.  PHIVE does not yet have the ability for
conditional installs (see
phar-io/phive#295 (comment)) so the
script must check the PHP version before running the update.

PHIVE has also been added to the tools (self-referencing) as it is not available
by default to GitHub Actions.

Note: There are warnings from PHPUnit 9.x about use of deprecated `assert`
methods (which will be removed in PHPUnit 10.x).  However, these don't cause the
tests to fail, and the replacement methods are not available in PHPUnit 7.x
which is still required to test against PHP 7.1 and 7.2.

Part of #925/#926.

.github/workflows/ci.yml

@@ -144,5 +144,11 @@ jobs:
           composer update --no-progress "${DEPENDENCIES}";
           composer show;
 
+      - name: Update PHIVE dependencies
+        run: |
+          if ! composer php:version | grep -q '^7\.[012]'; then
+            composer phive:update:phpunit
+          fi
+
       - name: Run Tests
         run: composer ci:tests:unit

composer.json

@@ -90,7 +90,8 @@
         "ci": [
             "@ci:static",
             "@ci:dynamic"
-        ]
+        ],
+        "phive:update:phpunit": "echo y | \"./tools/phive.phar\" update phpunit"
     },
     "extra": {
         "branch-alias": {

phive.xml

@@ -1,7 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <phive xmlns="https://phar.io/phive">
+  <phar name="phar-io/phive" version="~0.13.5" installed="0.13.5" location="./tools/phive.phar" copy="true"/>
   <phar name="php-cs-fixer" version="^2.16" installed="2.16.3" location="./tools/php-cs-fixer.phar" copy="true"/>
   <phar name="phpmd" version="^2.8" installed="2.8.2" location="./tools/phpmd.phar" copy="true"/>
-  <phar name="phpunit" version="^7.5" installed="7.5.20" location="./tools/phpunit.phar" copy="true"/>
+  <phar name="phpunit" version="^7.5.20 || ^9.4.3" installed="7.5.20" location="./tools/phpunit.phar" copy="true"/>
   <phar name="psalm" version="^3.11.5" installed="3.11.5" location="./tools/psalm.phar" copy="true"/>
 </phive>