expectedError doesn't read stderr

During running of a simple test:
```
schemaVersion: "2.0.0"
commandTests:
- name: "command not found"
command: "composer"
args: ["-v"]
expectedError: [".*executable file not found.*"]
```
I am getting:
```
=================================
====== Test file: php.yaml ======
=================================
=== RUN: Command Test: command not found
--- FAIL
Error: Error creating container: API error (400): OCI runtime create failed: container_linux.go:345: starting container process caused "exec: \"composer\": executable file not found in $PATH": unknown

=================================
============ RESULTS ============
=================================
Passes: 0
Failures: 1
Total tests: 1

FAIL
FATA[0001] FAIL
```
which is definitely incorrect.