doc: link monorepo to test laravel (#6530)

Author: soyuka

src/Laravel/README.md

@@ -3,7 +3,8 @@
 ## Tests
 
     cd src/Laravel
-    composer update --prefer-stable
+    composer global require soyuka/pmu
+    composer global link ../../
     vendor/bin/testbench workbench:build
     vendor/bin/testbench package:test
     # or
@@ -12,3 +13,8 @@
 The test server is also available through:
 
     vendor/bin/testbench serve
+
+A command is available to remove the database:
+
+    vendor/bin/testbench workbench:drop-sqlite-db
+