Run ChannelCrashTests on Linux/CI (#379)

When `ChannelCrashTests` was added, it was never configured to run on Linux/CI. This PR:

- Renames the test case to `ChannelConnectivityTests`
- Adds an entry to `LinuxMain` so this is run on CI
- Makes the class `final`
- Updates indentation to be consistent with the rest of the project (2 spaces)

Tests/LinuxMain.swift

@@ -21,6 +21,7 @@ XCTMain([
   // SwiftGRPC
   testCase(gRPCTests.allTests),
   testCase(ChannelArgumentTests.allTests),
+  testCase(ChannelConnectivityTests.allTests),
   testCase(ClientCancellingTests.allTests),
   testCase(ClientTestExample.allTests),
   testCase(ClientTimeoutTests.allTests),

Tests/SwiftGRPCTests/ChannelConnectivityTests.swift

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2018, gRPC Authors All rights reserved.
+ *
+ * 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.
+ */
+@testable import SwiftGRPC
+import XCTest
+
+final class ChannelConnectivityTests: BasicEchoTestCase {
+  override var defaultTimeout: TimeInterval { return 0.4 }
+
+  static var allTests: [(String, (ChannelConnectivityTests) -> () throws -> Void)] {
+    return [
+      ("testDanglingConnectivityObserversDontCrash", testDanglingConnectivityObserversDontCrash)
+    ]
+  }
+}
+
+extension ChannelConnectivityTests {
+  func testDanglingConnectivityObserversDontCrash() {
+    let completionHandlerExpectation = expectation(description: "completion handler called")
+
+    client?.channel.subscribe { connectivityState in
+      print("ConnectivityState: \(connectivityState)")
+    }
+
+    let request = Echo_EchoRequest(text: "foo bar baz foo bar baz")
+    _ = try! client!.expand(request) { callResult in
+      print("callResult.statusCode: \(callResult.statusCode)")
+      completionHandlerExpectation.fulfill()
+    }
+
+    DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(300)) {
+      self.client = nil // Deallocating the client
+    }
+
+    waitForExpectations(timeout: 0.5)
+  }
+}