added some more tests

[kevinpapst]

Description

• added some missing unit tests
• ignore AbstractMigration in Code Coverage, as it will be tested by the Migration runs (up and down) in CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I verified that my code applies to the guidelines (composer code-check)
• I updated the documentation (see here)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #1835 into master will increase coverage by 0.34%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1835      +/-   ##
============================================
+ Coverage     92.61%   92.95%   +0.34%     
+ Complexity     5161     5139      -22     
============================================
  Files           489      488       -1     
  Lines         15979    15937      -42     
============================================
+ Hits          14799    14815      +16     
+ Misses         1180     1122      -58     

Impacted Files	Coverage Δ	Complexity Δ
src/Saml/Provider/SamlProvider.php	100.00% <0.00%> (+14.28%)	6.00% <0.00%> (ø%)
src/Saml/SamlAuth.php	100.00% <0.00%> (+25.00%)	3.00% <0.00%> (ø%)
src/Saml/Controller/SamlController.php	78.26% <0.00%> (+52.17%)	9.00% <0.00%> (ø%)