[unittests] Use std::make_tuple to make some toolchains happy again

My toolchain stopped working (LLVM 8.0, libstdc++ 5.4.0) after 577adda5:

06:25:37 ../unittests/Support/Path.cpp:91:7: error: chosen constructor is explicit in copy-initialization
06:25:37       {"", false, false},  {"/", true, true},      {"/foo", true, true},
06:25:37       ^~~~~~~~~~~~~~~~~~
06:25:37 /proj/flexasic/app/llvm/8.0/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.4.0/../../../../include/c++/5.4.0/tuple:479:19: note: explicit constructor declared here
06:25:37         constexpr tuple(_UElements&&... __elements)
06:25:37                   ^

This commit adds explicit calls to std::make_tuple to work around
the problem.