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[3pp definition of build/fuchsia test scripts]

//build/ is an own repo (https://chromium.googlesource.com/chromium/src/build) which can be easily double-referred as a submodule. 3pp currently raises an error if a 3pp file is duplicated, see https://crrev.com/c/4658885. So instead of using //build/fuchsia/, place the 3pp file here to avoid the error.