Fix shrink tests on disks w/<15% free

The tests rely on the allocation decider allowing allocation and
the disk threshold decider wasn't allowing it. This came up from
time to in CI but *all* the time on my local machine because it is
a small ssd.

rest-api-spec/src/main/resources/rest-api-spec/test/indices.shrink/10_basic.yaml

@@ -1,8 +1,26 @@
+---
+setup:
+  # Disable the disk threshold decider so the test doesn't fail if the disk is
+  # > 85% full
+    - do:
+        cluster.put_settings:
+          body:
+            persistent:
+              cluster.routing.allocation.disk.threshold_enabled: false
+          flat_settings: true
+
+---
+teardown:
+  # Reset the disk threshold decider so we don't leave anything behind.
+    - do:
+        cluster.put_settings:
+          body:
+            persistent:
+              cluster.routing.allocation.disk.threshold_enabled: null
+          flat_settings: true
+
 ---
 "Shrink index via API":
-  - skip:
-      features: always
-      reason:  Fails consistently for Nik and sometimes for Jenkins. Skip until we can get it passing consistently.
   # creates an index with one document solely allocated on the master node
   # and shrinks it into a new index with a single shard
   # we don't do the relocation to a single node after the index is created