net: pin the user agent value and test the shared client

Build the shared OkHttpClient through UserAgent.httpClient() so the unit test exercises the same client the app uses, assert the exact 'Subtracks (Android; <version>)' value rather than the property it writes, and run the test under the Android runner.

app/src/test/kotlin/com/subtracks/UserAgentTest.kt

@@ -1,24 +1,29 @@
 package com.subtracks
 
-import okhttp3.OkHttpClient
+import androidx.test.ext.junit.runners.AndroidJUnit4
 import okhttp3.Request
 import okhttp3.mockwebserver.MockResponse
 import okhttp3.mockwebserver.MockWebServer
 import org.junit.Assert.assertEquals
 import org.junit.Test
+import org.junit.runner.RunWith
 
+@RunWith(AndroidJUnit4::class)
 class UserAgentTest {
     @Test
-    fun addsTheSubtracksUserAgent() {
+    fun theSharedClientSendsTheSubtracksUserAgent() {
         val server = MockWebServer()
         server.enqueue(MockResponse())
         server.start()
-        val client = OkHttpClient.Builder().addInterceptor(UserAgent.interceptor()).build()
 
-        client.newCall(Request.Builder().url(server.url("/")).build()).execute().close()
+        UserAgent
+            .httpClient()
+            .newCall(Request.Builder().url(server.url("/")).build())
+            .execute()
+            .close()
         val header = server.takeRequest().getHeader("User-Agent")
         server.shutdown()
 
-        assertEquals(UserAgent.value, header)
+        assertEquals("Subtracks (Android; ${BuildConfig.VERSION_NAME})", header)
     }
 }