[libc++abi][libunwind] Run c++abi and unwind tests against a fake install root (#110171)

This is what we started doing in libc++ and it straightens up a lot of
things that only happened to work before, notably the presence of
relative rpaths in dylibs when running from the build tree.

This unlocks the ability to link against a just-built dylib but run
against another version of the dylib (for example the system-provided
one), which is necessary for proper backdeployment testing.

This patch adds a lot of code duplication between the libc++ and
libc++abi testing setups. However, there is already a large amount of
duplication and the only real way to get rid of it is to merge libc++abi
into libc++. In a way, this patch is a step in that direction because it
closes the gap between the two libraries' testing setup.

GitOrigin-RevId: 2121b961fd8f1cf5449a73a20643db53431e5bc0