Fix: Use assertStringContainsString() instead of assertContains()

[localheinz]

This PR

• uses assertStringContainsString() instead of the deprecated assertContains()

[codecov]

Codecov Report

Merging #247 into 1.x will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                1.x     #247   +/-   ##
=========================================
  Coverage     78.97%   78.97%           
  Complexity       43       43           
=========================================
  Files             3        3           
  Lines           195      195           
=========================================
  Hits            154      154           
  Misses           41       41

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