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/node/context.cpp

@@ -7,6 +7,7 @@
 #include <addrman.h>
 #include <banman.h>
 #include <interfaces/chain.h>
+#include <kernel/context.h>
 #include <net.h>
 #include <net_processing.h>
 #include <netgroup.h>