🐛 fix endianness in DCX and XX_minus_YY gate matrix definition

[burgholzer]

Description

This has been brought up as in cda-tum/mqt-qcec/issues/489.
Turns out the endianness of the DCX gate matrix was wrong and this has not been detected because the tests where multiplying the gates of the definition for verification purposes in the wrong order.
After adjusting the tests, another incorrect endianness result popped up for the xx_minus_yy gate.

Both issues have now been fixed.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.1%. Comparing base (dcda5c1) to head (4961a2c).
Report is 129 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #741     +/-   ##
=======================================
- Coverage   92.2%   92.1%   -0.2%     
=======================================
  Files        125     125             
  Lines      13739   13770     +31     
  Branches    2158    2157      -1     
=======================================
+ Hits       12678   12684      +6     
- Misses      1061    1086     +25     

Flag	Coverage Δ
cpp	91.8% <100.0%> (-0.2%)	⬇️
python	99.7% <ø> (ø)

Files with missing lines	Coverage Δ
include/mqt-core/dd/GateMatrixDefinitions.hpp	100.0% <100.0%> (ø)

... and 16 files with indirect coverage changes