test(command): cover exec failure paths

Command tests did not assert behavior when exec fails in child processes.

I added deterministic regression coverage that forces execl/execlp failure and
verifies non-zero exit status propagation for both open() and forkExec paths.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

test/utils/meson.build

@@ -14,6 +14,7 @@ test_src = files(
     'JsonParser.cpp',
     'SafeSignal.cpp',
     'sleeper_thread.cpp',
+    'command.cpp',
     'css_reload_helper.cpp',
     '../../src/util/css_reload_helper.cpp',
 )