(PUP-11348) compare-then-set is actually slower

Always resetting the environment is faster the compare-then-set in cases where
the environment was changed and not.

```
$ cat bench.rb
require 'benchmark/ips'

Benchmark.ips do |x|
  unchanged = ENV.to_hash
  changed   = ENV.to_hash.merge!("foo" => "bar")

  x.report("test-set unchanged") { ENV.replace(unchanged) if ENV.to_hash != unchanged }
  x.report("test-set changed")   { ENV.replace(unchanged) if ENV.to_hash != changed }
  x.report("set unchanged")      { ENV.replace(unchanged) }
  x.report("set changed")        { ENV.replace(changed) }

  x.compare!
end

$ ruby --version
ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
$ ruby bench.rb
      Warming up --------------------------------------
    test-set unchanged     4.711k i/100ms
      test-set changed     2.691k i/100ms
        set unchanged     5.639k i/100ms
          set changed     5.480k i/100ms
  Calculating -------------------------------------
    test-set unchanged     46.809k (± 1.1%) i/s -    235.550k in   5.032833s
      test-set changed     26.749k (± 0.6%) i/s -    134.550k in   5.030353s
        set unchanged     55.451k (± 0.9%) i/s -    281.950k in   5.085048s
          set changed     54.355k (± 0.8%) i/s -    274.000k in   5.041297s

  Comparison:
        set unchanged:    55451.4 i/s
          set changed:    54354.6 i/s - 1.02x  slower
    test-set unchanged:    46808.7 i/s - 1.18x  slower
      test-set changed:    26748.7 i/s - 2.07x  slower
```

Author: joshcooper

lib/puppet/test/test_helper.rb

@@ -182,13 +182,8 @@ def self.after_each_test()
       end
       $saved_indirection_state = nil
 
-      # Restore the global process environment.  Can't just assign because this
-      # is a magic variable, sadly, and doesn't do that™.  It is sufficiently
-      # faster to use the compare-then-set model to avoid excessive work that it
-      # justifies the complexity.  --daniel 2012-03-15
-      if ENV.to_hash != $old_env
-        ENV.replace($old_env)
-      end
+      # Restore the global process environment.
+      ENV.replace($old_env)
 
       # Clear all environments
       Puppet.lookup(:environments).clear_all