custom-named-profiles: fix merges of predefined profiles
custom-named-profiles: fix merges of predefined profiles

The predefined 'test' and 'bench' profiles should exist even if
the user did not define anything in the Cargo.toml. This also
rewrites how the merging is done.

nipunn1313/cargo,rustyhorde/cargo,tshepang/cargo,rustyhorde/cargo,tshepang/cargo,rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,tshepang/cargo,nipunn1313/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,tshepang/cargo,vojtechkral/cargo,rustyhorde/cargo,tsurai/cargo,tsurai/cargo,rust-lang/cargo,vojtechkral/cargo