Normalize generated schemes to match Xcode's output

XcodeGen (through XcodeProj) serializes schemes slightly differently to
Xcode, so Xcode rewrites them as soon as it touches them (running tests,
switching test plans, opening the scheme editor), leaving recurring local
changes behind.

Add a small Swift script, run from XcodeGen's postGenCommand, that rewrites
the generated schemes the way Xcode saves them: it drops default-valued
attributes and empty CommandLineArguments, orders TestPlanReference
attributes, and moves TestPlans to the end of the TestAction, omitting the
redundant empty Testables.

Regenerated schemes are byte-identical to the copies saved by Xcode. The
script can be removed once https://github.com/tuist/XcodeProj/pull/1189
ships in XcodeGen.

ElementX.xcodeproj/xcshareddata/xcschemes/ShareExtension.xcscheme

@@ -5,8 +5,7 @@
    version = "1.7">
    <BuildAction
       parallelizeBuildables = "YES"
-      buildImplicitDependencies = "YES"
-      runPostActionsOnFailure = "NO">
+      buildImplicitDependencies = "YES">
       <BuildActionEntries>
          <BuildActionEntry
             buildForTesting = "YES"
@@ -28,8 +27,7 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
-      shouldUseLaunchSchemeArgsEnv = "YES"
-      onlyGenerateCoverageForSpecifiedTargets = "NO">
+      shouldUseLaunchSchemeArgsEnv = "YES">
       <MacroExpansion>
          <BuildableReference
             BuildableIdentifier = "primary"
@@ -41,8 +39,6 @@
       </MacroExpansion>
       <Testables>
       </Testables>
-      <CommandLineArguments>
-      </CommandLineArguments>
    </TestAction>
    <LaunchAction
       buildConfiguration = "Debug"
@@ -66,8 +62,6 @@
             ReferencedContainer = "container:ElementX.xcodeproj">
          </BuildableReference>
       </BuildableProductRunnable>
-      <CommandLineArguments>
-      </CommandLineArguments>
    </LaunchAction>
    <ProfileAction
       buildConfiguration = "Release"
@@ -85,8 +79,6 @@
             ReferencedContainer = "container:ElementX.xcodeproj">
          </BuildableReference>
       </BuildableProductRunnable>
-      <CommandLineArguments>
-      </CommandLineArguments>
    </ProfileAction>
    <AnalyzeAction
       buildConfiguration = "Debug">