Generate valid german phone numbers

[neeg]

fix for #1876

[codecov-io]

Codecov Report

Merging #1877 into master will decrease coverage by 0.57%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1877      +/-   ##
============================================
- Coverage     56.87%   56.29%   -0.58%     
- Complexity     2068     2069       +1     
============================================
  Files           306      307       +1     
  Lines          4851     4853       +2     
============================================
- Hits           2759     2732      -27     
- Misses         2092     2121      +29

Impacted Files	Coverage Δ	Complexity Δ
src/Faker/Provider/de_DE/PhoneNumber.php	100% <100%> (ø)	1 <1> (?)
src/Faker/Provider/en_IN/Person.php	0% <0%> (-100%)	1% <0%> (ø)
src/Faker/Provider/fi_FI/Address.php	33.33% <0%> (-66.67%)	3% <0%> (ø)
src/Faker/Provider/ko_KR/Company.php	50% <0%> (-50%)	2% <0%> (ø)
src/Faker/Provider/sv_SE/Address.php	50% <0%> (-50%)	4% <0%> (ø)
src/Faker/Provider/ro_MD/Address.php	65.21% <0%> (-34.79%)	10% <0%> (ø)
src/Faker/Provider/en_HK/Address.php	44.44% <0%> (-33.34%)	9% <0%> (ø)
src/Faker/Provider/sr_Latn_RS/Address.php	66.66% <0%> (-33.34%)	3% <0%> (ø)
src/Faker/Provider/sr_RS/Address.php	66.66% <0%> (-33.34%)	3% <0%> (ø)
src/Faker/Provider/sr_Cyrl_RS/Address.php	66.66% <0%> (-33.34%)	3% <0%> (ø)
... and 30 more

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[pimjansen]

Could you please add a valid test for this scenario too?