lib.debug.throwTestFailures: init

`lib.debug.runTests` provides a unit test evaluator for Nix, but its
results are returned in a raw and difficult-to-read form.

Currently, different callers output the results in various ways:
`builtins.throw (builtins.toJSON failures)` and `builtins.throw ("Tests
failed: " + lib.generators.toPretty { } failures)` are both used.

This change adds a new `lib.debug.throwTestFailures` function which
displays the results nicely before throwing an exception (or returns
`null` if no failures are given), unifying these disparate call-sites.

First, each failing test is pretty-printed in a `trace` message:

```
trace: FAIL testDerivation:
  Expected: <derivation a>
    Result: <derivation b>
```

Then, an exception is thrown containing the number of tests that failed
(and their names), followed by the raw JSON of the results (for parity
with previous usage, and because `lib.generators.toPretty` sometimes
omits information that `builins.toJSON` includes):

```
error:
       … while evaluating the file '...':

       … caused by explicit throw
         at /nix/store/.../lib/debug.nix:528:7:
          527|       in
          528|       throw (
             |       ^
          529|         builtins.seq traceFailures (

       error: 1 tests failed:
       - testDerivation

       [{"expected":"/nix/store/xh7kyqp69mxkwspmi81a94m9xx74r8dr-a","name":"testDerivation","result":"/nix/store/503l84nir4zw57d1shfhai25bxxn16c6-b"}]
```

lib/tests/misc.nix

@@ -4741,8 +4741,6 @@ runTests {
     expected = "/non-existent/this/does/not/exist/for/real/please-dont-mess-with-your-local-fs/default.nix";
   };
 
-  # Tests for cross index utilities
-
   testRenameCrossIndexFrom = {
     expr = lib.renameCrossIndexFrom "pkgs" {
       pkgsBuildBuild = "dummy-build-build";
@@ -4819,4 +4817,31 @@ runTests {
     };
   };
 
+  testThrowTestFailuresEmpty = {
+    expr = lib.debug.throwTestFailures {
+      failures = [ ];
+    };
+
+    expected = null;
+  };
+
+  testThrowTestFailures = testingThrow (
+    lib.debug.throwTestFailures {
+      failures = [
+        {
+          name = "testDerivation";
+          expected = builtins.derivation {
+            name = "a";
+            builder = "bash";
+            system = "x86_64-linux";
+          };
+          result = builtins.derivation {
+            name = "b";
+            builder = "bash";
+            system = "x86_64-linux";
+          };
+        }
+      ];
+    }
+  );
 }