Attempt to stabilize the test suite

[garious]

The integration tests are allowed to open sockets, so running them
in parallel may cause "Too many open files" errors. This patch
runs the unit tests in parallel and the integration test serially.

[garious]

@carllin, fyi

[garious]

@rob-solana, can you provide a portable command to get the filenames without extensions into cargo with --test= prefixes? Preferably that appeases shellcheck.

Current patch causes this error on Ubuntu:

find: Must separate multiple arguments to -type using: ','
--
  | error: Found argument '--' which wasn't expected, or isn't valid in this context
  |  
  | USAGE:
  | cargo test --jobs <N> --verbose

[rob-solana]

working on it