Invalid certificate PEM is returned when x509.CreateCertificate fails

I analyzed `tls.go` under the library threat model: the exported `GenerateTestCertificate(host)` is the trust boundary, and `host` is the main externally influenced input. The function has no fixed-size buffers, unsafe memory operations, goroutines, external commands, or slice indexing, so I do not see a Go memory-safety zero-day here. The main risks are input-validation/resource-consumption issues and an error-handling footgun.

Key observations:

- `host` is copied directly into `cert.DNSNames` without length, DNS syntax, or wildcard validation. A very long attacker-influenced host can cause allocation and encoding work proportional to the input. A malformed or wildcard hostname can produce a nonconforming or overly broad SAN if the generated certificate is used outside a controlled test context.
- Every call generates a 2048-bit RSA key. If this exported helper is reachable from a request path or otherwise invoked repeatedly with untrusted input, it becomes a CPU/memory DoS surface.
- If `x509.CreateCertificate` fails, the code still PEM-encodes `certBytes`, which may be nil/invalid, and returns a non-nil `CERTIFICATE` PEM block alongside the error. A caller that misuses the return values or ignores the error may handle an invalid certificate object.
- The certificate is generated as a CA (`IsCA: true`) with a 365-day validity period. This is expected for a test helper, but it would be a lifecycle/trust issue if accidentally deployed to production. I did not list that as a primary finding because it is more a usage/design concern than a directly triggerable code vulnerability.

```json
[
  {
    "severity": "low",
    "title": "Unvalidated host input allows unbounded certificate material and nonconforming SANs",
    "function": "GenerateTestCertificate",
    "description": "The host parameter is placed directly into cert.DNSNames without length or DNS-name validation. An attacker-influenced very long host can cause DER/PEM allocation and processing proportional to the input, while invalid or wildcard hostnames can produce nonconforming or overly broad subject alternative names if the generated certificate is used outside tests."
  },
  {
    "severity": "low",
    "title": "Expensive RSA-2048 key generation on every call is a DoS surface",
    "function": "GenerateTestCertificate",
    "description": "Each invocation generates a 2048-bit RSA private key and a certificate. If this public helper is called with attacker-influenced input in a request path or without rate limiting, an external attacker can trigger repeated CPU- and memory-intensive key generation."
  },
  {
    "severity": "low",
    "title": "Invalid certificate PEM is returned when x509.CreateCertificate fails",
    "function": "GenerateTestCertificate",
    "description": "certBytes is encoded into a PEM block even if x509.CreateCertificate returns an error. The function then returns a non-nil, syntactically present but invalid CERTIFICATE PEM alongside the error. Callers that misuse the return values or ignore the error may use an invalid certificate; on failure the certificate return value should be nil."
  }
]
```