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/eval-fail-assert.err.exp

@@ -0,0 +1,36 @@
+error:
+       … while evaluating the attribute 'body'
+
+         at /pwd/lang/eval-fail-assert.nix:4:3:
+
+            3|
+            4|   body = x "x";
+             |   ^
+            5| }
+
+       … from call site
+
+         at /pwd/lang/eval-fail-assert.nix:4:10:
+
+            3|
+            4|   body = x "x";
+             |          ^
+            5| }
+
+       … while calling 'x'
+
+         at /pwd/lang/eval-fail-assert.nix:2:7:
+
+            1| let {
+            2|   x = arg: assert arg == "y"; 123;
+             |       ^
+            3|
+
+       error: assertion '(arg == "y")' failed
+
+       at /pwd/lang/eval-fail-assert.nix:2:12:
+
+            1| let {
+            2|   x = arg: assert arg == "y"; 123;
+             |            ^
+            3|