Add missing unwrap() in tests introduced in 4abfd515e554bef90c25dbe

[TheBlueMatt]

#513 introduced a new compiler warning.

[codecov]

Codecov Report

Merging #543 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #543   +/-   ##
=======================================
  Coverage   90.24%   90.24%           
=======================================
  Files          34       34           
  Lines       19206    19206           
=======================================
  Hits        17332    17332           
  Misses       1874     1874           

Impacted Files	Coverage Δ
lightning/src/ln/functional_tests.rs	96.33% <100.00%> (ø)

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[TheBlueMatt]

This is absurdly trivial.