BibLaTeX long URL extending into border despite using biburllcpenalty

You could insert a bit stretchable space:

\documentclass[paper=A4,fontsize=12pt]{scrartcl}
\usepackage[left=3cm, right=3cm, bottom=3cm, top=3cm]{geometry}
\usepackage{fontspec}

\usepackage[backend=biber,style=authoryear,citestyle=authoryear]{biblatex}
\DeclareNameAlias{sortname}{last-first}
\DeclareNameAlias{default}{last-first}
\setcounter{biburllcpenalty}{100}
\setcounter{biburlucpenalty}{100}
\setcounter{biburlnumpenalty}{100}
\addbibresource{resource.bib}

\renewcommand*{\biburlsetup}{%
  \Urlmuskip=0mu plus 3mu\relax
  \mathchardef\UrlBigBreakPenalty=100\relax
  \mathchardef\UrlBreakPenalty=200\relax
  \def\UrlBigBreaks{\do\:\do\-}%
  \def\UrlBreaks{%
    \do\.\do\@\do\/\do\\\do\!\do\_\do\|\do\;\do\>\do\]\do\)\do\}%
    \do\,\do\?\do\'\do\+\do\=\do\#\do\$\do\&\do\*\do\^\do\"}%
  \ifnumgreater{\value{biburlnumpenalty}}{0}
    {\def\do##1{\appto\UrlSpecials{\do##1{\mathchar`##1 \mskip 0mu plus 1mu\penalty\value{biburlnumpenalty}}}}%
     \do\1\do\2\do\3\do\4\do\5\do\6\do\7\do\8\do\9\do\0}
    {}%
  \ifnumgreater{\value{biburlucpenalty}}{0}
    {\def\do##1{\appto\UrlSpecials{\do##1{\mathchar`##1 \mskip 0mu plus 1mu\penalty\value{biburlucpenalty}}}}%
     \do\A\do\B\do\C\do\D\do\E\do\F\do\G\do\H\do\I\do\J
     \do\K\do\L\do\M\do\N\do\O\do\P\do\Q\do\R\do\S\do\T
     \do\U\do\V\do\W\do\X\do\Y\do\Z}
    {}%
  \ifnumgreater{\value{biburllcpenalty}}{0}
    {\def\do##1{\appto\UrlSpecials{\do##1{\mathchar`##1 \mskip 0mu plus 1mu\penalty\value{biburllcpenalty}}}}%
     \do\a\do\b\do\c\do\d\do\e\do\f\do\g\do\h\do\i\do\j
     \do\k\do\l\do\m\do\n\do\o\do\p\do\q\do\r\do\s\do\t
     \do\u\do\v\do\w\do\x\do\y\do\z}
    {}%
  \let\do=\noexpand}

\begin{document}


\cite{Test.2018}
\printbibliography
\end{document}

try

\usepackage[style=authoryear,citestyle=authoryear]{biblatex}
\usepackage{xurl}

biblatex 3.13 implements a way to add stretchable space to the URL in the way demonstrated in Ulrike Fischer's answer, see https://github.com/plk/biblatex/issues/850.

The relevant lengths are \biburlnumskip, \biburlucskip and \biburllcskip.

\documentclass[paper=A4,fontsize=12pt]{scrartcl}
\usepackage[left=3cm, right=3cm, bottom=3cm, top=3cm]{geometry}
\usepackage{fontspec}

\usepackage[backend=biber,style=authoryear]{biblatex}

\DeclareNameAlias{default}{family-given}
\DeclareNameAlias{sortname}{default}

\setcounter{biburllcpenalty}{100}
\setcounter{biburlucpenalty}{100}
\setcounter{biburlnumpenalty}{100}

\biburlnumskip=0mu plus 1mu\relax
\biburlucskip=0mu plus 1mu\relax
\biburllcskip=0mu plus 1mu\relax

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@misc{Test.2018,
  author = {{testauthor test author testauthor}},
  year   = {2018},
  title  = {{testfile test file testfile}},
  url    = {http://abcdefghijklmnopqrstuvwxy.com/phrrFK5dS1ztTjiaCmveRARXpGBtV01U%20B93FcrxkOVhj2CceIguNuUPpwDaUD4GjTTWh9cL30D6rLgWxapYlhegDXQGFlE0hao7B2YS227ca%20keoWHWNmQVKVcNOUFxfsCp0SwCElfTVtpMv6DKqiSwhWgO2zrHvstd4UNBo99UQBSCTkzL7lD2UyRLTRy6xXdJ}
}
\end{filecontents}

\addbibresource{\jobname.bib}

\begin{document}
\cite{Test.2018}
\printbibliography
\end{document}