[7.0] Run game tests without downloading assets

[rubensworks]

It would be nice if there was a way to skip downloading game assets such as sounds when running game tests, as the assets are not necessary for running the game tests.

This used to be possible in ForgeGradle 6.x using ./gradlew runGameTestServer -x downloadAssets

This would solve issues with running game tests in GitHub Actions, as I'm sometimes encountering CI failures such as the following:

...
  Downloading missing asset: minecraft/sounds/mob/chicken/picky/ambient7.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/picky/ambient8.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/picky/death.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/picky/hurt1.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/picky/hurt2.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/picky/hurt3.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/plop.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/say1.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/say2.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/say3.ogg
  Downloading missing asset: minecraft/sounds/mob/chicken/step1.ogg
Error: Exception in thread "main" java.lang.IllegalStateException: Failed to download minecraft/sounds/mob/chicken/step1.ogg
	at net.minecraftforge.launcher.DownloadAssets.download(DownloadAssets.java:57)
	at net.minecraftforge.launcher.Main.run(Main.java:116)
	at net.minecraftforge.launcher.Main.main(Main.java:34)
Caused by: java.io.IOException: https://resources.download.minecraft.net/e1/e16352150262ab49686f6c0aeaffa7532d3157ea
	at net.minecraftforge.launcher.shadow.net.minecraftforge.util.download.DownloadUtils.downloadFile(DownloadUtils.java:98)
	at net.minecraftforge.launcher.DownloadAssets.download(DownloadAssets.java:50)
	... 2 more
Caused by: java.net.http.HttpTimeoutException: request timed out
	at java.net.http/jdk.internal.net.http.HttpClientImpl.send(HttpClientImpl.java:921)
	at java.net.http/jdk.internal.net.http.HttpClientFacade.send(HttpClientFacade.java:133)
	at net.minecraftforge.launcher.shadow.net.minecraftforge.util.download.DownloadUtilsImpl.connect(DownloadUtilsImpl.java:55)
	at net.minecraftforge.launcher.shadow.net.minecraftforge.util.download.DownloadUtils.downloadFile(DownloadUtils.java:92)
	... 3 more