PYTHON-2554 Temp workaround for PYTHON-2931/PYTHON-1864

Author: ShaneHarvey

test/crud/unified/aggregate-write-readPreference.json

@@ -237,7 +237,7 @@
                     }
                   ],
                   "$readPreference": {
-                    "$$exists": false
+                    "mode": "primary"
                   },
                   "readConcern": {
                     "level": "local"
@@ -425,7 +425,7 @@
                     }
                   ],
                   "$readPreference": {
-                    "$$exists": false
+                    "mode": "primary"
                   },
                   "readConcern": {
                     "level": "local"

test/crud/unified/db-aggregate-write-readPreference.json

@@ -222,7 +222,7 @@
                     }
                   ],
                   "$readPreference": {
-                    "$$exists": false
+                    "mode": "primary"
                   },
                   "readConcern": {
                     "level": "local"
@@ -416,7 +416,7 @@
                     }
                   ],
                   "$readPreference": {
-                    "$$exists": false
+                    "mode": "primary"
                   },
                   "readConcern": {
                     "level": "local"