Use `Implicit Types` in HeapLang tests; this fixes compilation with Coq 8.11.

tests/heap_lang.ref

@@ -195,7 +195,7 @@ Tactic failure: wp_pure: cannot find ?y in (Var "x") or
   Σ : gFunctors
   heapG0 : heapG Σ
   E1, E2 : coPset
-  P : iPropI Σ
+  P : iProp Σ
   ============================
   Atomic (stuckness_to_atomicity NotStuck) (#() #())
 

tests/heap_lang.v

@@ -325,6 +325,7 @@ End inv_mapsto_tests.
 
 Section atomic.
   Context `{!heapG Σ}.
+  Implicit Types P Q : iProp Σ.
 
   (* These tests check if a side-condition for [Atomic] is generated *)
   Check "wp_iMod_fupd_atomic".