[nextest-runner] use an IndexMap for JUnit aggregation

[sunshowers]

Test suites that run first are now stored first in the JUnit output. Previously this was effectively in completely random order, which doesn't seem right.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.62%. Comparing base (be72863) to head (ac3c337).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/reporter/aggregator/junit.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1974   +/-   ##
=======================================
  Coverage   80.61%   80.62%           
=======================================
  Files          95       95           
  Lines       23071    23071           
=======================================
+ Hits        18599    18600    +1     
+ Misses       4472     4471    -1     

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