Allow re-use of vmtest output
When developing or debugging vmtest unittest failures of collected data
it's useful to skip the install/boot/collect phases and just re-run the
unittest against a directory of collected output.
Introduce CURTIN_VMTEST_REUSE_TOPDIR setting to enable this. If the testcase
directory is present then vmtest will skip vmlaunch/boot/collect stages. If the
directory is missing, the test runs as usual.
CURTIN_VMTEST_REUSE_TOPDIR=1 ./tools/jenkins-runner \
tests/vmtests/test_basic.py:XenialTestBasic