Improve test suite by adding "composer run multicover"

This will aid on running the tests with coverage on both PHP7 and PHP8,
merge coverage reports and render them as HTML to be inspected by
humans.

It's not completely maintenance free because some paths will probably
have to be adjusted for others, but it's way better than nothing.

DEVELOP.rst

@@ -81,15 +81,15 @@ Installation
 
 Install prerequisites::
 
-    # Install different PHP releases.
-    brew install php@7.3 php@7.4 php@8.0 composer
+    # Install different PHP releases and Composer.
+    brew install php@7.3 php@7.4 php@8.0 brew-php-switcher composer
 
     # Select PHP version.
     brew-php-switcher 7.3
     brew-php-switcher 7.4
     brew-php-switcher 8.0
 
-    # Install xdebug extension for tracking code coverage.
+    # Install `xdebug` extension into each environment for tracking code coverage.
     pecl install xdebug
 
 Get the sources::
@@ -113,9 +113,19 @@ Running the Tests
         --mount type=bind,source=$PWD/test/provisioning/truststore,target=/vagrant/test/provisioning/truststore \
         --publish 4200:4200 --publish 5432:5432 crate/crate:nightly
 
-    # Run test suite
+    # Run test suite on current/default version of PHP
     composer run test
 
+    # Run tests on both PHP7 and PHP8 to get the full picture of coverage
+    composer run multicover
+    open build/multicover/html/index.html
+
+
+Invoke code style checks
+========================
+
+::
+
     # Run code style checks
     composer run check-style
 

composer.json

@@ -25,7 +25,8 @@
     "require-dev": {
         "phpunit/phpunit": "^9.5",
         "phpstan/phpstan": "^0.12",
-        "squizlabs/php_codesniffer": "^3.5"
+        "squizlabs/php_codesniffer": "^3.5",
+        "nimut/phpunit-merger": "^1.1"
     },
     "autoload-dev": {
         "psr-0": {
@@ -35,6 +36,8 @@
     },
     "scripts": {
         "test": "XDEBUG_MODE=coverage phpunit --coverage-clover build/logs/clover.xml",
+        "coverage-html": "XDEBUG_MODE=coverage phpunit --coverage-html build/logs/html",
+        "multicover": "./devtools/php-multicover.sh",
         "check-style": "phpcs",
         "fix-style": "phpcbf"
     }