Use !canImport(ObjectiveC) instead of !os(macOS) (#1916)

There are a bunch of platforms which are not macOS and still have the
ObjC runtime. This also brings this check inline with the
`generate-linuxmain` command.

See rdar://problem/46739400

Author: neonichu

Sources/Workspace/InitPackage.swift

@@ -367,7 +367,7 @@ public final class InitPackage {
             stream <<< """
                 import XCTest
 
-                #if !os(macOS)
+                #if !canImport(ObjectiveC)
                 public func allTests() -> [XCTestCaseEntry] {
                     return [
                         testCase(\(moduleName)Tests.allTests),