Support running tests from different build directories

The tests that run exec-fd-leak-checker expect the binary to be located
in the current directory. This is not always the case; for instance, the
binaries could be built under `tests`, but be invoked under the
top-level build directory.

We can use an environment variable to control what's the location of the
test binaries, and fall back to the current directory if the variable is
unset.

Reviewed-by: Daniel Stone <daniels@collabora.com>