allocator: deflake full disk test

In cockroachdb#97409 we introduced cluster settings to control the disk fullness
threshold for rebalancing towards a store and shedding replicas off of
the store. The `TestAllocatorFullDisks` assumes the total number of
range bytes is equal or less than the rebalance threshold of the nodes,
however the test was updated to use the shed threshold instead. This
caused the test to flake occasionally as there was more than the
expected amount of total range bytes.

This patch changes the ranges per node calculation to use the rebalance
threshold again, instead of the shed threshold

Fixes: cockroachdb#100033

Release note: None

pkg/kv/kvserver/allocator/allocatorimpl/allocator_test.go

@@ -8475,7 +8475,14 @@ func TestAllocatorFullDisks(t *testing.T) {
 
 	do := makeDiskCapacityOptions(&st.SV)
 
-	rangesPerNode := int(math.Floor(capacity * do.ShedAndBlockAllThreshold / rangeSize))
+	// Each range is equally sized (16mb), we want the number of ranges per node,
+	// when their size is added, to be no greater than the full disk rebalance
+	// threshold (0.925%) e.g for below:
+	//   capacity  = 1024mb
+	//   rangeSize = 16mb
+	//   threshold = 0.925
+	//   rangesPerNode =  ⌊1024mb * 0.925 / 16mb⌋ = 59
+	rangesPerNode := int(math.Floor(capacity * do.RebalanceToThreshold / rangeSize))
 	rangesToAdd := rangesPerNode * nodes
 
 	// Initialize testStores.