• download the server jar from here https://piston-data.mojang.com/v1/objects/e003d151668a0eff64c1191972707655e341f8f5/server.jar
• create a datapack (or download my example) and place it in a testpacks folder next to the jar. (need to unzip it to avoid running into MC-279522)
• run "java -DbundlerMainClass="net.minecraft.gametest.Main" -jar server.jar --packs testpacks --report=test.xml"
• observe that the output only runs the one inbuild test, not your custom one

code analysis: 
GameTestMainUtil#runGameTestServer only loads the inbuild datapack

ServerPacksSource.createVanillaTrustedRepository() 

but should load the datapacks from the world

ServerPacksSource.createPackRepository(levelStorageAccess)