Add a target runner for running tests in VMs (#5285)

This PR allows us to run tests inside falcon VMs via nextest. Due to the
lack of [nextest per-test target
runners](nextest-rs/nextest#1358), we enable
the target runner via environment variable and run a specific test at a
time.

In order to run a test, do the following, where the trailing
`--nocapture` is optional but encouraged at this moment.

```
cd omicron
CARGO_TARGET_X86_64_UNKNOWN_ILLUMOS_RUNNER=./test-utils/src/bin/falcon_runner.sh cargo nextest run -p <package> <test name> --nocapture
```


An example: 
```
CARGO_TARGET_X86_64_UNKNOWN_ILLUMOS_RUNNER=./test-utils/src/bin/falcon_runner.sh cargo nextest run -p omicron-test-utils launch --nocapture
```

A few things remain to be done, although this is safe to merge without
adding them in this PR. See `test-utils/README.md` for details.

.gitignore

@@ -14,4 +14,6 @@ rusty-tags.vi
 *.sw*
 tags
 .direnv
+.falcon/*
+.img/*
 connectivity-report.json