fix: return type for postgres tests template

[silaselisha]

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

Problem/Feature

Postgres mustStartPostgresContainer expects to return a function of type func(ctx context.Context, opts ...testcontainers.TerminateOption) error but func(context.Context) error is returned.

Description of Changes:

• Wrong type is returned for postgres test, hence changed the postgres template configuration to ensure that the right type is returned when generating postgres test file.

Checklist

• I have self-reviewed the changes being requested
• I have updated postgres.tmpl (check issue ticket [Bug] Wrong type is returned for postgres test #363)

[Ujstor]

LGTM