tests: Write out LD_LIBRARY_PATH for built shared libraries

When a test executable references a local shared library, make sure that
we apply the appropriate $LD_LIBRARY_PATH so that the linker can find it
at runtime.

The DT_RUNPATH entry does ensure that the binary references the path to
the shared library build, however the RUNPATH list is only searched
after $LD_LIBRARY_PATH. So if the user has a shared library of the same
name in their $LD_LIBRARY_PATH, this will be the version found and used
for running the test. This is bad if you're trying to use Meson to test
a shared library you're developing and have installed in a local prefix
which is under $LD_LIBRARY_PATH.

Fixes #1635

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