Test harness improvements and refactorings (#147)

- feature: the 0.2.0 implementation has a new test mode, accessible on
  `-T` which runs test eu scripts or test targets within scripts. It
  is currently an undocumented draft implementation which is not yet
  documented but it is used to run the eucalypt test harness.

- feature: The tester can run all tests in a directory, using `eu -T
  <dir>` in which case the directory is also now added to the library
  path so tests can easily import relative paths.

- break: The initial YAML implementation does not support outputting
  tags, because the underlying Rust library used does not. This is a
  temporary regression pending a switch of library to restore tag
  output. The current test does not detect the issue.

Author: gmorpheme

.github/workflows/build-rust.yaml

@@ -133,7 +133,7 @@ jobs:
       - name: run final test with release binary
         run: |
           target/release/eu_amd64 -v
-          target/release/eu_amd64 harness/test -T -L harness
+          target/release/eu_amd64 harness/test -T
       - name: Generate changelog
         run: |
           echo "$(git-changelog latest...)" > CHANGELOG.md
@@ -196,7 +196,7 @@ jobs:
       - name: Run final test with release binary
         run: |
           target/release/eu_darwin -v
-          target/release/eu_darwin harness/test -T -L harness
+          target/release/eu_darwin harness/test -T
       - name: Upload binary
         uses: actions/upload-artifact@v1
         with: