For consistency with other tests that use example public and private
keys, replace use of `concat!` macro with `include_str!`. This way, key
data embedded into source with single line of code.
A new context is now created by calling Context::new and therefore you
always have a valid context. This is much more in Rust style and will
allow a lot of furture simplifications on the context itself.
The FFI layer has not yet been adjusted in this commit and thus will
fail.
This marks the function safe and returns Result, it also now returns
the ConfiguredAddr since it has to look this up anyway and it makes
testing more easy. Turns out it reduces some duplicate SQL query in
some callers too.
More test code has been moved from dc_imex to test_utils as it's
more genrally applicable.
