Add unit tests for libexpr (#5377)

* libexpr: fix builtins.split example

The example was previously indicating that multiple whitespaces would be
collapsed into a single captured whitespace. That isn't true and was
likely a mistake when being documented initially.

* Fix segfault on unitilized list when printing value

Since lists are just chunks of memory the individual elements in the
list might be unitilized when a programming error happens within Nix.

In this case the values are null-initialized (at least with Boehm GC)
and we can avoid a nullptr deref when printing them.

I ran into this issue while ensuring that new expression tests would
show the actual value on an assertion failure.

This is unlikely to cause any runtime performance regressions as
printing values is not really in the hot path (unless the repl is the
primary use case).

* Add operator<< for ValueTypes

* Add libexpr tests

This introduces tests for libexpr that evalulate various trivial Nix
language expressions and primop invocations that should be good smoke
tests wheter or not the implementation is behaving as expected.

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