core: fix beginning of line in iterated regexec (closes #1521)

[sim642]

Besides the original issue in eval_replace_regex I found three more iterated regexec calls where similar issue can be observed. I added tests for all four.

[codecov-commenter]

Codecov Report

Merging #1523 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1523   +/-   ##
=======================================
  Coverage   35.04%   35.05%           
=======================================
  Files         204      204           
  Lines       84373    84382    +9     
=======================================
+ Hits        29571    29581   +10     
+ Misses      54802    54801    -1     

Impacted Files	Coverage Δ
src/core/wee-eval.c	91.96% <100.00%> (+0.01%)	⬆️
src/core/wee-string.c	88.53% <100.00%> (+0.08%)	⬆️
src/gui/gui-color.c	80.24% <100.00%> (+0.04%)	⬆️
src/plugins/buflist/buflist-bar-item.c	12.64% <0.00%> (-0.05%)	⬇️
src/gui/gui-chat.c	70.62% <0.00%> (+0.20%)	⬆️

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