Changes the tests to not run the demo program

[xxxserxxx]

The test suite was mostly testing the demo program; one test (TestUnregisteredRootCommand) only tested demo program behavior, without executing any checks on the core library.

This PR replaces all command-executions in tests with programmatic equivalents; it also removes the test for the demo program -- if that program were tested, it'd be best done in demo/cmd_test.go, not in the library.

Benchmarks show an improvement from 8.6 seconds per run to 500 milliseconds per run, or a 17x speed improvement. More importantly, the tests are more directly testing the library, not the demo program. Improvements are still possible; these tests are functional tests rather than unit tests.

Benchmark for pre-PR:

hyperfine 'go test . ; go clean -testcache' 
Benchmark 1: go test . ; go clean -testcache
  Time (mean ± σ):      8.601 s ±  2.788 s    [User: 10.870 s, System: 5.040 s]
  Range (min … max):    5.799 s … 13.011 s    10 runs

Post-PR:

10220(tests)⚡ » hyperfine 'go test . ; go clean -testcache' 
Benchmark 1: go test . ; go clean -testcache
  Time (mean ± σ):     503.5 ms ± 160.1 ms    [User: 849.9 ms, System: 325.8 ms]
  Range (min … max):   353.0 ms … 901.0 ms    10 runs