Fix tests running with older versions of PHPUnit

tests/before_script.sh

@@ -20,6 +20,10 @@ phpunit_v7_5_shim() {
     remove_expectWarning
 }
 
+phpunit_v8_1_shim() {
+    remove_expectWarning
+}
+
 start_php_servers() {
     for i in $(seq 0 6); do
         port=8000
@@ -39,11 +43,27 @@ composer install --prefer-source --no-interaction
 # Let test server know we should allow testing.
 export PHP_CURL_CLASS_TEST_MODE_ENABLED="yes"
 
-if [[ "${CI_PHP_VERSION}" == "7.0" ]]; then
+# Determine which phpunit to use.
+if [[ -f "../vendor/bin/phpunit" ]]; then
+    phpunit_to_use="../vendor/bin/phpunit"
+else
+    phpunit_to_use="phpunit"
+fi
+
+phpunit_version="$("${phpunit_to_use}" --version | grep -Eo "[0-9]+\.[0-9]+\.[0-9]+")"
+echo "${phpunit_version}"
+
+if [[ "${phpunit_version}" == "6.5."* ]]; then
     phpunit_v6_5_shim
+elif [[ "${phpunit_version}" == "7.5."* ]]; then
+    phpunit_v7_5_shim
+elif [[ "${phpunit_version}" == "8.1."* ]]; then
+    phpunit_v8_1_shim
+fi
+
+if [[ "${CI_PHP_VERSION}" == "7.0" ]]; then
     start_php_servers
 elif [[ "${CI_PHP_VERSION}" == "7.1" ]]; then
-    phpunit_v7_5_shim
     start_php_servers
 elif [[ "${CI_PHP_VERSION}" == "7.2" ]]; then
     start_php_servers