[lldb/Test] Fix ASan/TSan workaround for Xcode Python 3

The Python 3 interpreter in Xcode has a relative RPATH and dyld fails to
load it when we copy it into the build directory.

This patch adds an additional check that the copied binary can be
executed. If it doesn't, we assume we're dealing with the Xcode python
interpreter and return the path to the real executable. That is
sufficient for the sanitizers because only system binaries need to be
copied to work around SIP.

This patch also moves all that logic out of LLDBTest and into the lit
configuration so that it's executed only once per test run, instead of
once for every test. Although I didn't benchmark the difference this
should result in a mild speedup.

Differential revision: https://reviews.llvm.org/D81696

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