[FAB-6529] Fix new CA unit-tests
[FAB-6529] Fix new CA unit-tests

This CR adds to the unit-tests calls to ca.closeDB
so that the DB file can effectively be removed, and
error checking to catch any problems with the removal
of test files.

With these changes the unit-tests pass:

$ go test -v
=== RUN   TestCABadCACertificates
2017/10/10 14:00:54 [INFO] The CA key and certificate already exist
2017/10/10 14:00:54 [INFO] The key is stored by BCCSP provider 'SW'
...
2017/10/10 14:03:14 [DEBUG] Closing server DBs
2017/10/10 14:03:15 [DEBUG] Stop: successful stop on port 7075
--- PASS: TestRegisterationAffiliation (1.81s)
=== RUN   TestEnd
--- PASS: TestEnd (0.14s)
=== RUN   TestSRVServerClean
--- PASS: TestSRVServerClean (0.03s)
PASS
ok  	github.com/hyperledger/fabric-ca/lib	141.073s

Patch-set #2: rebase

Change-Id: Iea4b8afce63cc1bf7a8c100322d2736ffd056156
Signed-off-by: Arnaud J Le Hors <336b2f4fb14e0a6aba0291ba81894e387297584e@us.ibm.com>

saadk10/fabric-ca,saadk10/fabric-ca