Fix Testables element ordering

[maxwellE]

Testables should come after MacroExpansion (Xcode expects this and
will edit projects with invalid order).

[codecov-commenter]

Codecov Report

Merging #702 (cb72bed) into main (e02240f) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #702      +/-   ##
==========================================
- Coverage   84.92%   84.83%   -0.09%     
==========================================
  Files         157      157              
  Lines        9109     9109              
==========================================
- Hits         7736     7728       -8     
- Misses       1373     1381       +8     

Impacted Files	Coverage Δ
Sources/XcodeProj/Scheme/XCScheme+TestAction.swift	91.03% <100.00%> (ø)
Sources/XcodeProj/Errors/Errors.swift	36.11% <0.00%> (-11.12%)	⬇️

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[kwridan]

Thanks @maxwellE

Verified via inspecting a manually created project in Xcode and selecting a "Expand Variables Based On" target within the test action

<TestAction
      buildConfiguration = "Debug"
      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
      shouldUseLaunchSchemeArgsEnv = "NO">
      <MacroExpansion>
         <BuildableReference
            BuildableIdentifier = "primary"
            BlueprintIdentifier = "FBA856212875AC7B007ABE1A"
            BuildableName = "AsyncTrials.framework"
            BlueprintName = "AsyncTrials"
            ReferencedContainer = "container:AsyncTrials.xcodeproj">
         </BuildableReference>
      </MacroExpansion>
      <Testables>
         <TestableReference
            skipped = "NO">
            <BuildableReference
               BuildableIdentifier = "primary"
               BlueprintIdentifier = "FBA856292875AC7C007ABE1A"
               BuildableName = "AsyncTrialsTests.xctest"
               BlueprintName = "AsyncTrialsTests"
               ReferencedContainer = "container:AsyncTrials.xcodeproj">
            </BuildableReference>
         </TestableReference>
      </Testables>
   </TestAction>

[luispadron]

Thanks!