Set a fixed locale in RealSubscriptionsManagerTest (#7064)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1202552961248957/task/1211852239322423?focus=true

### Description

### Steps to test this PR

- [x] Tests should pass
- [ ]

### No UI changes

Author: lmac012

common/common-test/src/main/java/com/duckduckgo/common/test/FixedLocaleRule.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2025 DuckDuckGo
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.duckduckgo.common.test
+
+import org.junit.rules.TestRule
+import org.junit.runner.Description
+import org.junit.runners.model.Statement
+import java.util.Locale
+
+class FixedLocaleRule(private val locale: Locale = Locale.US) : TestRule {
+
+    override fun apply(base: Statement, description: Description) = object : Statement() {
+        override fun evaluate() {
+            val originalLocale = Locale.getDefault()
+            Locale.setDefault(locale)
+            try {
+                base.evaluate()
+            } finally {
+                Locale.setDefault(originalLocale)
+            }
+        }
+    }
+}

subscriptions/subscriptions-impl/src/test/java/com/duckduckgo/subscriptions/impl/RealSubscriptionsManagerTest.kt

@@ -10,6 +10,7 @@ import com.android.billingclient.api.ProductDetails.SubscriptionOfferDetails
 import com.android.billingclient.api.PurchaseHistoryRecord
 import com.duckduckgo.autofill.api.email.EmailManager
 import com.duckduckgo.common.test.CoroutineTestRule
+import com.duckduckgo.common.test.FixedLocaleRule
 import com.duckduckgo.common.utils.CurrentTimeProvider
 import com.duckduckgo.feature.toggles.api.FakeFeatureToggleFactory
 import com.duckduckgo.feature.toggles.api.Toggle.State
@@ -108,6 +109,9 @@ class RealSubscriptionsManagerTest(private val authApiV2Enabled: Boolean) {
     @get:Rule
     val coroutineRule = CoroutineTestRule()
 
+    @get:Rule
+    val fixedLocalRule = FixedLocaleRule()
+
     private val authService: AuthService = mock()
     private val subscriptionsService: SubscriptionsService = mock()
     private val authDataStore: FakeSubscriptionsDataStore = FakeSubscriptionsDataStore()