rune: Improve test case macros and docs #1857

Job	Run time
docs	3m 28s
clippy	2m 10s
msrv	1m 33s
nightly	5m 28s
rustfmt	14s
basics	1s
test_miri (rune)	3m 51s
build_feature (alloc)	3m 43s
test_miri (rune-core)	1m 4s
test_miri (rune-alloc)	5m 57s
build_feature (cli)	3m 57s
test_doc	6m 24s
no_std (ubuntu-latest, minimal)	4m 3s
build_feature (cli,doc)	4m 3s
no_std (macos-latest, minimal)	2m 11s
test_rune	2m 56s
build_feature (cli,fmt)	4m 6s
wasm	1m 7s
build_feature (doc)	3m 41s
build_feature (workspace)	3m 49s
test_lib	4m 23s
build_feature (languageserver)	3m 59s
build_feature (byte-code)	3m 27s
build_feature (capture-io)	3m 23s
build_feature (emit)	3m 28s
	1h 22m 26s

Provide feedback