Add SkipTestMain utility function for clearer output

[bbrks]

This change makes it much more obvious that the package's tests are being completely skipped.

Formatting details can be negotiated. I tried to match the regular test output format for any parsers that may be interpreting this, however that may be more trouble than it's worth.

There's also no special exit code that we can return other than 0 for TestMain that would show skipped instead of ok. Any non-zero code shows FAIL.

Before

$ go test -v -count=1 -run '^TestAutoImportUserXattrNoSyncData$' ./rest/importuserxattrtest
ok  	github.com/couchbase/sync_gateway/rest/importuserxattrtest	0.574s

After

$ go test -v -count=1 -run '^TestAutoImportUserXattrNoSyncData$' ./rest/importuserxattrtest
=== RUN   TestMain
    main_test.go:24: user xattr tests require xattrs and EE
--- SKIP: TestMain (0.00s)
ok  	github.com/couchbase/sync_gateway/rest/importuserxattrtest	0.582s

[torcolvin]

This seems great, I've definitely been confused by this.

sync_gateway/db/indextest/main_test.go

Line 26 in ca9728d

return

sync_gateway/rest/importtest/main_test.go

Line 22 in ca9728d

if !base.TestUseXattrs() { // import tests only run if xattrs are enabled

should get added too.