ci: run cargo test

[tisonkun]

also move e2e/ to the root path

tests/ holds all Rust (integration) tests, doesn't mess up things

ready for review if tests passed.

[codecov-commenter]

Codecov Report

Merging #11 (2f6c237) into master (39232e5) will decrease coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
- Coverage   85.15%   84.94%   -0.22%     
==========================================
  Files           9        9              
  Lines         283      279       -4     
==========================================
- Hits          241      237       -4     
  Misses         42       42              

Impacted Files	Coverage Δ
src/reporter/grpc.rs	0.00% <ø> (ø)
tests/propagation.rs	96.77% <100.00%> (ø)
tests/trace_context.rs	100.00% <100.00%> (ø)

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[tisonkun]

cc @Shikugawa

[Shikugawa]

NIce cleanup! Thanks!