Add more tests for serialization of field syntax

[djaglowski]

Field syntax can be used to refer directly to the body of a log,
but the serialization of this case was not well tested. This
change adds additional tests to cover valid cases.

[codecov]

Codecov Report

Merging #350 (235d7f9) into main (d1c9d7a) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #350     +/-   ##
=======================================
- Coverage   77.2%   77.1%   -0.1%     
=======================================
  Files         94      94             
  Lines       4471    4471             
=======================================
- Hits        3452    3451      -1     
- Misses       699     701      +2     
+ Partials     320     319      -1     

Impacted Files	Coverage Δ
...perator/builtin/transformer/recombine/recombine.go	74.3% <0.0%> (-2.1%)	⬇️
operator/builtin/input/tcp/tcp.go	79.6% <0.0%> (+1.6%)	⬆️