test(pkgs_spec): fix rubocop violations (--enable-pending-cops)

```
Offenses:

test/integration/default/controls/pkgs_spec.rb:27:5: W: [Corrected] Lint/SymbolConversion:
Unnecessary symbol conversion; use iotop: instead.
    'iotop': ''
    ^^^^^^^
test/integration/default/controls/pkgs_spec.rb:33:5: W: [Corrected] Lint/SymbolConversion:
Unnecessary symbol conversion; use alien: instead.
    'alien': '8.95-8.fc29',
    ^^^^^^^
test/integration/default/controls/pkgs_spec.rb:34:5: W: [Corrected] Lint/SymbolConversion:
Unnecessary symbol conversion; use iotop: instead.
    'iotop': '0.6-18.fc29'
    ^^^^^^^
test/integration/default/controls/pkgs_spec.rb:46:5: W: [Corrected] Lint/SymbolConversion:
Unnecessary symbol conversion; use alien: instead.
    'alien': '8.95',
    ^^^^^^^
test/integration/default/controls/pkgs_spec.rb:48:5: W: [Corrected] Lint/SymbolConversion:
Unnecessary symbol conversion; use iotop: instead.
    'iotop': '0.6-'
    ^^^^^^^
```

test/integration/default/controls/pkgs_spec.rb

@@ -24,14 +24,14 @@
   held_packages = {
     # We use this test for held packages in a list,
     # with no version (current version).
-    'iotop': ''
+    iotop: ''
   }
   lock_file = '/etc/yum/pluginconf.d/versionlock.list'
 when 'fedora'
   platform_packages = ['python3-dnf-plugin-versionlock']
   held_packages = {
-    'alien': '8.95-8.fc29',
-    'iotop': '0.6-18.fc29'
+    alien: '8.95-8.fc29',
+    iotop: '0.6-18.fc29'
   }
   lock_file = '/etc/dnf/plugins/versionlock.list'
 # Adding empty Suse entries, to get tests passing
@@ -43,9 +43,9 @@
 when 'debian'
   platform_packages = %w[]
   held_packages = {
-    'alien': '8.95',
+    alien: '8.95',
     # To match also ubuntu16's
-    'iotop': '0.6-'
+    iotop: '0.6-'
   }
   lock_file = '/var/lib/dpkg/status'
 when 'linux'