qemu-iotests: move command line and environment handling from TestRunner to TestEnv

qemu-iotests: move command line and environment handling from TestRunner to TestEnv

In the next patch, "check" will learn how to execute a test script without
going through TestRunner.  To enable this, keep only the text output
and subprocess handling in the TestRunner; move into TestEnv the logic
to prepare for running a subprocess.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210323181928.311862-4-pbonzini@redhat.com>
Message-Id: <20210503110110.476887-4-pbonzini@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>