lib/tests: move throwTestFailures tests to standalone script

The throwTestFailures tests in misc.nix produce confusing trace output
that looks like test failures even when tests pass.

This commit moves these tests to a new lib/tests/debug.sh script,
following the pattern from modules.sh, sources.sh, and filesystem.sh.

The confusing trace output does not appear in any logs anymore.

Example of the confusing log:

trace: FAIL "testDerivation":
Expected:
<derivation a>

Result:
<derivation b>

evaluation warning: Using `lib.generators.toPlist` without `escape = true` is deprecated
[ ]

lib/tests/debug.sh

@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+
+# Tests lib/debug.nix
+# Run:
+# [nixpkgs]$ lib/tests/debug.sh
+# or:
+# [nixpkgs]$ nix-build lib/tests/release.nix
+
+set -euo pipefail
+shopt -s inherit_errexit
+
+# Use
+#     || die
+die() {
+  echo >&2 "test case failed: " "$@"
+  exit 1
+}
+
+if test -n "${TEST_LIB:-}"; then
+  NIX_PATH=nixpkgs="$(dirname "$TEST_LIB")"
+else
+  NIX_PATH=nixpkgs="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.."; pwd)"
+fi
+export NIX_PATH
+
+work="$(mktemp -d)"
+clean_up() {
+  rm -rf "$work"
+}
+trap clean_up EXIT
+cd "$work"
+
+expectSuccess() {
+    local expr=$1
+    local expectedResultRegex=$2
+    if ! result=$(nix-instantiate --eval --strict --json \
+        --expr "with (import <nixpkgs/lib>).debug; $expr" 2>/dev/null); then
+        die "$expr failed to evaluate, but it was expected to succeed"
+    fi
+    if [[ ! "$result" =~ $expectedResultRegex ]]; then
+        die "$expr == $result, but $expectedResultRegex was expected"
+    fi
+}
+
+expectFailure() {
+    local expr=$1
+    local expectedErrorRegex=$2
+    if result=$(nix-instantiate --eval --strict --json 2>"$work/stderr" \
+        --expr "with (import <nixpkgs/lib>).debug; $expr"); then
+        die "$expr evaluated successfully to $result, but it was expected to fail"
+    fi
+    if [[ ! "$(<"$work/stderr")" =~ $expectedErrorRegex ]]; then
+        die "Error was $(<"$work/stderr"), but $expectedErrorRegex was expected"
+    fi
+}
+
+# Test throwTestFailures with empty failures list
+expectSuccess 'throwTestFailures { failures = [ ]; }' "null"
+
+# Test throwTestFailures with actual failures
+# This should throw with a specific error message format
+expectFailure '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";
+      };
+    }
+  ];
+}' "1 tests failed"
+
+echo >&2 tests ok

lib/tests/misc.nix

@@ -6,7 +6,8 @@
   which is `throw`'s and `abort`'s, without error messages.
 
   If you need to test error messages or more complex evaluations, see
-  `lib/tests/modules.sh`, `lib/tests/sources.sh` or `lib/tests/filesystem.sh` as examples.
+  `lib/tests/modules.sh`, `lib/tests/sources.sh`, `lib/tests/filesystem.sh` or
+  `lib/tests/debug.sh` as examples.
 
   To run these tests:
 
@@ -4813,32 +4814,4 @@ runTests {
       targetTarget = "prefix-tt";
     };
   };
-
-  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";
-          };
-        }
-      ];
-    }
-  );
 }

lib/tests/test-with-nix.nix

@@ -60,6 +60,9 @@ pkgs.runCommand "nixpkgs-lib-tests-nix-${nix.version}"
     echo "Running lib/tests/sources.sh"
     TEST_LIB=$PWD/lib bash lib/tests/sources.sh
 
+    echo "Running lib/tests/debug.sh"
+    TEST_LIB=$PWD/lib bash lib/tests/debug.sh
+
     echo "Running lib/tests/network.sh"
     TEST_LIB=$PWD/lib bash lib/tests/network.sh