Use named config method

And mock the preference value, rather than setting it. Before, the set preference could have leaked to other tests.
(I noticed that it was already like this in product_import_spec.rb)

app/services/weights_and_measures.rb

@@ -67,6 +67,6 @@ def find_largest_unit(scales, product_scale_system)
   end
 
   def available_units
-    Spree::Config.preferences[:available_units].split(",")
+    Spree::Config.available_units.split(",")
   end
 end

spec/models/spree/line_item_spec.rb

@@ -695,12 +695,9 @@ module Spree
         let(:p2) { create(:product, name: 'Clear United States Honey', variant_unit_scale: 453.6) }
         let(:v2) { create(:variant, product: p2, unit_value: 453.6) }
         let(:li2) { create(:line_item, order: o, product: p2, variant: v2) }
-        let(:available_units) {
-          ["mg", "g", "kg", "T", "oz", "lb", "mL", "cL", "dL", "L", "kL", "gal"].join(",")
-        }
 
         before do
-          Spree::Config.set_preference(:available_units, available_units)
+          allow(Spree::Config).to receive(:available_units).and_return("g,lb,oz,kg,T,mL,L,kL")
         end
 
         it "returns options_text" do
@@ -709,7 +706,7 @@ module Spree
           expect(li.unit_to_display).to eq("ponies")
         end
 
-        it "returns options_text based on units" do
+        it "returns options_text based on configured units" do
           expect(li1.options_text).to eq("500g")
           expect(li2.options_text).to eq("1lb")
         end

spec/services/unit_prices_spec.rb

@@ -6,13 +6,10 @@
   subject { UnitPrice.new(variant) }
   let(:variant) { Spree::Variant.new }
   let(:product) { instance_double(Spree::Product) }
-  let(:available_units) {
-    ["mg", "g", "kg", "T", "oz", "lb", "mL", "cL", "dL", "L", "kL", "gal"].join(",")
-  }
 
   before do
     allow(variant).to receive(:product) { product }
-    Spree::Config.set_preference(:available_units, available_units)
+    allow(Spree::Config).to receive(:available_units).and_return("g,lb,oz,kg,T,mL,L,kL")
   end
 
   describe "#unit" do

spec/services/variant_units/option_value_namer_spec.rb

@@ -4,13 +4,6 @@
 
 module VariantUnits
   describe OptionValueNamer do
-    let(:available_units) {
-      ["mg", "g", "kg", "T", "oz", "lb", "mL", "cL", "dL", "L", "kL", "gal"].join(",")
-    }
-
-    before do
-      Spree::Config.set_preference(:available_units, available_units)
-    end
     describe "generating option value name" do
       let(:v) { Spree::Variant.new }
       let(:p) { Spree::Product.new }
@@ -69,6 +62,10 @@ module VariantUnits
       let(:v) { Spree::Variant.new }
       let(:subject) { OptionValueNamer.new v }
 
+      before do
+        allow(Spree::Config).to receive(:available_units).and_return("g,lb,oz,kg,T,mL,L,kL")
+      end
+
       it "generates simple values" do
         p = double(:product, variant_unit: 'weight', variant_unit_scale: 1.0)
         allow(v).to receive(:product) { p }

spec/services/weights_and_measures_spec.rb

@@ -12,7 +12,7 @@
 
   before do
     allow(variant).to receive(:product) { product }
-    Spree::Config.set_preference(:available_units, available_units)
+    allow(Spree::Config).to receive(:available_units).and_return(available_units)
   end
 
   describe "#system" do

spec/system/admin/product_import_spec.rb

@@ -53,14 +53,6 @@
 
   let(:shipping_category_id_str) { Spree::ShippingCategory.all.first.id.to_s }
 
-  let(:available_units) {
-    ["mg", "g", "kg", "T", "oz", "lb", "mL", "cL", "dL", "L", "kL", "gal"].join(",")
-  }
-
-  before do
-    Spree::Config.set_preference(:available_units, available_units)
-  end
-
   describe "when importing products from uploaded file" do
     before do
       allow(Spree::Config).to receive(:available_units).and_return("g,lb,oz,kg,T,mL,L,kL")