Report test command output paths on failure

testing/baselines/tests.install-force-test-fail/out

@@ -1,6 +1,6 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 === install without skiptests
-error: failed to run tests for one<...>/foo: test_command failed with exit code 1
+error: failed to run tests for one<...>/foo: test_command failed with exit code 1; stdout: <...>/zkg.test_command.stdout; stderr: <...>/zkg.test_command.stderr
 Running unit tests for "one<...>/foo"
 have_load_foo=0
 === install with skiptests

testing/baselines/tests.upgrade-test-fail/out

@@ -3,7 +3,7 @@ Installing "one<...>/foo"
 Installed "one<...>/foo" (1.0.2)
 Loaded "one<...>/foo"
 === upgrade without skiptests
-error: failed to run tests for one<...>/foo: test_command failed with exit code 1
+error: failed to run tests for one<...>/foo: test_command failed with exit code 1; stdout: <...>/zkg.test_command.stdout; stderr: <...>/zkg.test_command.stderr
 Running unit tests for "one<...>/foo"
 === upgrade with skiptest
 Upgraded "one<...>/foo" (1.0.3)

zeekpkg/manager.py

@@ -2783,7 +2783,8 @@ def test(
             if rc != 0:
                 assert stage.state_dir
                 return (
-                    f"test_command failed with exit code {rc}",
+                    f"test_command failed with exit code {rc}; "
+                    f"stdout: {outfile}; stderr: {errfile}",
                     False,
                     stage.state_dir,
                 )