backends: allow LD_LIBRARY_PATH to be overwritten in test environments

[lzcunt]

This patch makes meson not touch LD_LIBRARY_PATH in test environments if it was set or unset. This is useful for writing test cases that validate behaviour of or otherwise depend on LD_LIBRARY_PATH, e.g. for mlibc's RTLD.

I'm not sure if anything out there relies on LD_LIBRARY_PATH being prepended by meson AND manually sets LD_LIBRARY_PATH, this could affect such use cases.

This was shortly discussed on IRC.

[lzcunt]

Turns out this breaks a test case (which depends on the exact opposite behavior that this patch implements), and I believe implementing this feature this way would be a breaking change that might actually affect projects. Help around how this might be implemented is appreciated. I'm thinking maybe a new method on environment() that "freezes" an environment variable (so neither meson nor anything else can append, prepend, set or unset it)