test: Add more testcases for previously untested code

[Swatinem]

Tests added:

• Object cloning and overwriting keys.
• Serializing more msgpack values.
• Forcing captured events to have a uuid event_id.
• JSON escapes and surrogate parsing, also fixing a bug for that.
• Discarding events when missing consent.
• Test and fix recording errors on current session.
• Sessions with username did.
• Lots of abnormal sessions on startup.

Bugs fixed:

• Wrong JSON decoding with surrogate pairs.
• Memory Leak parsing unmatched surrogate pairs.
• Never recorded error counts on sessions.

[codecov-commenter]

Codecov Report

Merging #317 into master will increase coverage by 2.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
+ Coverage   85.03%   87.18%   +2.15%     
==========================================
  Files          49       49              
  Lines        3975     3971       -4     
==========================================
+ Hits         3380     3462      +82     
+ Misses        595      509      -86     

Impacted Files	Coverage Δ
src/sentry_core.c	94.40% <100.00%> (+3.49%)	⬆️
src/sentry_json.c	88.13% <100.00%> (+12.70%)	⬆️
src/sentry_path.h	20.00% <0.00%> (-20.00%)	⬇️
src/symbolizer/sentry_symbolizer_unix.c	91.66% <0.00%> (-8.34%)	⬇️
src/sentry_options.c	77.55% <0.00%> (-0.23%)	⬇️
src/sentry_sync.h	97.29% <0.00%> (-0.08%)	⬇️
src/sentry_utils.c	90.07% <0.00%> (-0.04%)	⬇️
src/sentry_scope.c	98.26% <0.00%> (+0.57%)	⬆️
src/sentry_session.c	90.32% <0.00%> (+1.93%)	⬆️
src/sentry_database.c	82.26% <0.00%> (+2.12%)	⬆️
... and 5 more

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[Swatinem]

The failing test (memory leak) is totally expected and is fixed in #304