Expanded test suite

* Lang now verifies errors and parse output

* Some new miscellaneous tests

* Easy way to update the tests

* Document workflow in manual

* Use `!` not `~` as separater char for sed

  It is confusing to use `~` when we are talking about paths and home
  directories!

* Test test suite itself (`test/lang-test/infra.sh`)

Additionally, run shellcheck on `tests/lang.sh` to help ensure it is
correct, now that is is more complex.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

tests/lang/framework.sh

@@ -0,0 +1,33 @@
+# Golden test support
+#
+# Test that the output of the given test matches what is expected. If
+# `_NIX_TEST_ACCEPT` is non-empty also update the expected output so
+# that next time the test succeeds.
+function diffAndAcceptInner() {
+    local -r testName=$1
+    local -r got="$2"
+    local -r expected="$3"
+
+    # Absence of expected file indicates empty output expected.
+    if test -e "$expected"; then
+        local -r expectedOrEmpty="$expected"
+    else
+        local -r expectedOrEmpty=lang/empty.exp
+    fi
+
+    # Diff so we get a nice message
+    if ! diff "$got" "$expectedOrEmpty"; then
+        echo "FAIL: evaluation result of $testName not as expected"
+        badDiff=1
+    fi
+
+    # Update expected if `_NIX_TEST_ACCEPT` is non-empty.
+    if test -n "${_NIX_TEST_ACCEPT-}"; then
+        cp "$got" "$expected"
+        # Delete empty expected files to avoid bloating the repo with
+        # empty files.
+        if ! test -s "$expected"; then
+            rm "$expected"
+        fi
+    fi
+}