Factor out a bit more language testings infra

Will be used in a second test after `lang.sh`.

tests/functional/characterisation/framework.sh

@@ -0,0 +1,77 @@
+# shellcheck shell=bash
+
+# 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="$expected"
+    else
+        local -r expectedOrEmpty=characterisation/empty
+    fi
+
+    # Diff so we get a nice message
+    if ! diff --color=always --unified "$expectedOrEmpty" "$got"; then
+        echo "FAIL: evaluation result of $testName not as expected"
+        # shellcheck disable=SC2034
+        badDiff=1
+    fi
+
+    # Update expected if `_NIX_TEST_ACCEPT` is non-empty.
+    if test -n "${_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
+}
+
+function characterisationTestExit() {
+    # Make sure shellcheck knows all these will be defined by the caller
+    : "${badDiff?} ${badExitCode?}"
+
+    if test -n "${_NIX_TEST_ACCEPT-}"; then
+        if (( "$badDiff" )); then
+            set +x
+            echo 'Output did mot match, but accepted output as the persisted expected output.'
+            echo 'That means the next time the tests are run, they should pass.'
+            set -x
+        else
+            set +x
+            echo 'NOTE: Environment variable _NIX_TEST_ACCEPT is defined,'
+            echo 'indicating the unexpected output should be accepted as the expected output going forward,'
+            echo 'but no tests had unexpected output so there was no expected output to update.'
+            set -x
+        fi
+        if (( "$badExitCode" )); then
+            exit "$badExitCode"
+        else
+            skipTest "regenerating golden masters"
+        fi
+    else
+        if (( "$badDiff" )); then
+            set +x
+            echo ''
+            echo 'You can rerun this test with:'
+            echo ''
+            echo "    _NIX_TEST_ACCEPT=1 make tests/functional/${TEST_NAME}.test"
+            echo ''
+            echo 'to regenerate the files containing the expected output,'
+            echo 'and then view the git diff to decide whether a change is'
+            echo 'good/intentional or bad/unintentional.'
+            echo 'If the diff contains arbitrary or impure information,'
+            echo 'please improve the normalization that the test applies to the output.'
+            set -x
+        fi
+        exit $(( "$badExitCode" + "$badDiff" ))
+    fi
+}