Set the environment variable ADT_TEST_SDK_PATH

It is treated as the user having selected the given SDK root, and in
particular it means the wizard won't be shown even if you've wiped
adtUsed=true from your ~/.android/ddms.cfg etc.

[1] https://android.googlesource.com/platform/sdk/+/292eefb8faa2f75ddbc5d6e20084c9f9a762da29%5E!/