[develop2] test for relocatable toolchains

[memsharded]

Changelog: omit
Docs: omit

Close #5059

This test allows:

• To create relocatable toolchain packages
• A relocatable toolchain package requires some customization in every machine, that would break immutability
• We an use a base immutable package with the heavy bits, that take long to build
• Another package tool_requires this one, and uses build_policy="missing" and upload_policy="skip" to avoid uploading, and to build it when necessary, in a transparent way
• This other package implements in its package() or build() or both the necessary customizations to the current machine, copying from the other package what it needs

[AbrilRBS]

Also TIL that you can specify upload_policy!