Add execute_with to TestExternalities (#3793)

This function executes the given closure in a context where the test
externalities are set. This makes the srml tests easier to write, as the
test externalities need to be created anyway.

substrate/core/sr-primitives/src/lib.rs

@@ -73,9 +73,6 @@ pub use sr_arithmetic::helpers_128bit;
 /// Re-export big_uint stiff.
 pub use sr_arithmetic::biguint;
 
-#[cfg(feature = "std")]
-pub use externalities::set_and_run_with_externalities;
-
 /// An abstraction over justification for a block's validity under a consensus algorithm.
 ///
 /// Essentially a finality proof. The exact formulation will vary between consensus