Refactor lexer tests (#498)

- Refactor the lexer tests to be more readable, abandoning the
  previous string-based summary DSL in favor of a more obvious
  sequence of `TokenKinds` with optional lexemes. The new tests
  also test that token lexemes are correct.

- Move duplicated `unindent` function into a shared crate,
  `test-utilities`. This new versionless dev-dependency will
  prevent publishing to crates.io, at least until rust-lang/cargo/pull/7333
  makes it into stable. If we publish a new version before then,
  test-utilities will need to be published to crates.io, so we can depend
  on it by version.

.gitignore

@@ -1,7 +1,9 @@
-/target
 /.vagrant
 /README.html
-/tmp
-/fuzz/target
-/fuzz/corpus
 /fuzz/artifacts
+/fuzz/corpus
+/fuzz/target
+/target
+/test-utilities/Cargo.lock
+/test-utilities/target
+/tmp

Cargo.toml

@@ -37,3 +37,9 @@ features = ["termination"]
 [dev-dependencies]
 executable-path   = "1"
 pretty_assertions = "0.6"
+
+# Until github.com/rust-lang/cargo/pull/7333 makes it into stable,
+# this version-less dev-dependency will interfere with publishing
+# to crates.io.
+[dev-dependencies.test-utilities]
+path = "test-utilities"