Just test PoC

[redsun82]

This is a rough PoC showcasing just as a builder/test runner.

After installing just you can (in this PoC <language> can only be rust):

• just install <language> to install a language pack in-tree <language> can be omitted if running from inside its directory.
• just build <language> will build target/intree/codeql-<language> from the internal repository, if available, otherwise it will fall back to install.
• just test TESTS... FLAGS... will run the provided tests, if they are of the same kind (integration or QL tests), passing FLAGS to the runner (flags with arguments are required to be passed as --flag=value or -fvalue, as one string). For QL tests the appropriate build <language> is run. Notably, for QL tests this command works also if using codeql standalone from the internal repository (using install and codeql from PATH). If running from within a test directory, TESTS can be omitted and defaults to all tests in the current directory. Even the test recipe can be omitted, as it's the default.
• rust is a just module, so you can also run just rust ... with some other goodies
• b, i, t aliases

Examples:

just  # <- prints available recipes, could be made to do fuzzy selection
just install rust
(cd rust; just install)
(cd rust; just generate)
(cd rust; just)  # <- defaults to codegen + install
just rust  # <- same as above
just build rust  # or just rust build
(cd rust; just build)
just test rust/ql/test/extractor-tests/{File,utf8}
just test rust/ql/integration-tests/hello-*
(cd rust/ql/test/extractor-tests/File; just test)
(cd rust/ql/test/extractor-tests/File; just)  # <- defaults to test
(cd rust/ql/integration-tests/hello-project; just test)
(cd rust/ql/integration-tests/hello-project; just)  # <- defaults to test
just rust language-tests  # <- run all rust language tests
just rust integration-tests  # <- run all rust integration tests