Add test hook for retrieving ConfigSourceSupplier values immediately

[bbaldino]

See #23

[codecov]

Codecov Report

Merging #24 into master will increase coverage by 1.45%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master      #24      +/-   ##
============================================
+ Coverage     75.78%   77.24%   +1.45%     
  Complexity       46       46              
============================================
  Files            18       17       -1     
  Lines           190      189       -1     
  Branches         14       13       -1     
============================================
+ Hits            144      146       +2     
+ Misses           40       36       -4     
- Partials          6        7       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../jitsi/metaconfig/supplier/ConfigSourceSupplier.kt	88.00% <33.33%> (-7.46%)	7.00 <0.00> (ø)
.../kotlin/org/jitsi/metaconfig/MetaconfigSettings.kt	61.53% <100.00%> (+3.20%)	1.00 <0.00> (ø)

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