test: allow overriding DSN when launching demo from Xcode

[armcknight]

Avoids having to change it in code and potentially having to go back and forth in PRs like in #103

This is a debug-time complement to #115 which would allow doing this in the running app without relaunching.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.812%. Comparing base (b1a505d) to head (8832e8a).
⚠️ Report is 23 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##            master      #116       +/-   ##
=============================================
- Coverage   37.866%   35.812%   -2.055%     
=============================================
  Files           10        10               
  Lines         1022      1022               
  Branches       330       332        +2     
=============================================
- Hits           387       366       -21     
- Misses         628       650       +22     
+ Partials         7         6        -1     

Files with missing lines	Coverage Δ
EmpowerPlant/AppDelegate.swift	63.095% <100.000%> (ø)

... and 1 file with indirect coverage changes

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