test: shorten test trace file names

Make sure we don't generate extremely long file names for test trace log
file, as this can cause path-too-long errors. As the compilers in the
android ndk are deeply nested, it's very easy to trigger these.

I chose to output at most 4 path components -- this should keep the full
path for common cases like /usr/bin/gcc with room to spare, and should
be enough to uniquely identify the compiler for more deeply nested
cases.

llvm-svn: 296870

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