tests: move session joining tests to their own process

[mathstuf]

Fixes: #39

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.78%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   79.09%   79.88%   +0.78%     
==========================================
  Files          35       38       +3     
  Lines        2330     2371      +41     
==========================================
+ Hits         1843     1894      +51     
+ Misses        487      477      -10     

Impacted Files	Coverage Δ
tests/join_anonymous_session.rs	100.00% <100.00%> (ø)
tests/join_existing_named_session.rs	100.00% <100.00%> (ø)
tests/join_new_named_session.rs	100.00% <100.00%> (ø)
src/tests/utils/kernel.rs	87.09% <0.00%> (-0.21%)	⬇️
src/tests/search.rs	100.00% <0.00%> (ø)
src/tests/instantiate.rs	100.00% <0.00%> (ø)
src/api.rs	48.39% <0.00%> (+0.73%)	⬆️
... and 1 more

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