Add basic test for coverage example and library

[Porges]

The coverage code is not currently exercised by any test.

• Add a test to the coverage example so that it can run in PR builds.
• Specify --all-targets so that example tests are run.
• Install and use nextest instead of the standard test runner.
  • This will parallelize test runs across binaries so the overall test run is faster.
• Make sleep duration of agent configurable and reduce it in the test run so that it doesn't wait for 30 seconds.

[codecov-commenter]

Codecov Report

Merging #3035 (a11f5e8) into main (bd2b109) will increase coverage by 1.94%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3035      +/-   ##
==========================================
+ Coverage   29.43%   31.37%   +1.94%     
==========================================
  Files         305      274      -31     
  Lines       36544    30924    -5620     
==========================================
- Hits        10755     9702    -1053     
+ Misses      25789    21222    -4567     

Impacted Files	Coverage Δ
src/agent/onefuzz-agent/src/agent.rs	55.81% <100.00%> (ø)
src/agent/onefuzz-agent/src/agent/tests.rs	100.00% <100.00%> (ø)

... and 62 files with indirect coverage changes

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[tevoinea]

I love nextest, we should add it to the devcontainer config as well