init: Initialize globals with kernel::Context's life

...instead of explicitly calling init::{Set,Unset}Globals.

Cool thing about this is that in both the testing and bitcoin-chainstate
codepaths, we no longer need to explicitly unset globals. The
kernel::Context goes out of scope and the globals are unset
"automatically".

Also construct kernel::Context outside of AppInitSanityChecks()

src/init.h

@@ -19,6 +19,9 @@ class ArgsManager;
 namespace interfaces {
 struct BlockAndHeaderTipInfo;
 }
+namespace kernel {
+struct Context;
+}
 namespace node {
 struct NodeContext;
 } // namespace node