Fix compilation error if expression length != message length

[cschreib]

Reported in #158. The *REQUIRE_THAT macros were assuming that SNITCH_MAX_MESSAGE_LENGTH and SNITCH_MAX_EXPR_LENGTH were the same. If they weren't, this would cause a compilation error. The tests have now been modified so none of the lengths are identical, to catch future instances of this problem.

Also fixes #164.

[cschreib]

I still need to update the approval test data. But first I need to fix another issue, which was highlighted by these test failures: in the XML or TeamCity reporters, we need to escape some of the strings before reporting them. When the resulting string is too long and truncated, there is a risk of truncating halfway through an escaped sequence, which would render the escaped string invalid. I opened the bug ticket #164 to capture this.

I'm working on a solution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.05%. Comparing base (ccd3afa) to head (65ee518).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   93.87%   94.05%   +0.18%     
==========================================
  Files          27       27              
  Lines        1664     1698      +34     
==========================================
+ Hits         1562     1597      +35     
+ Misses        102      101       -1     

Files	Coverage Δ
include/snitch/snitch_registry.hpp	82.85% <ø> (ø)
include/snitch/snitch_string_utility.hpp	100.00% <ø> (ø)
src/snitch_reporter_catch2_xml.cpp	94.59% <100.00%> (+0.07%)	⬆️
src/snitch_reporter_teamcity.cpp	100.00% <100.00%> (ø)
src/snitch_string_utility.cpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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