MacOS: Convert openssl test FAIL to SKIP

[rfjakob]

On MacOS, building and testing without openssl is much easier. The tests should skip tests that fail because of missing openssl instead of failing:

$ ./test-without-openssl.bash 
gocryptfs v1.4-19-gab787e1 without_openssl; go-fuse v20170619; 2017-07-01 go1.8.3
gocryptfs v1.4-19-gab787e1; go-fuse v20170619; 2017-07-01 go1.8.3
ok  	github.com/rfjakob/gocryptfs	0.003s
?   	github.com/rfjakob/gocryptfs/gocryptfs-xray	[no test files]
ok  	github.com/rfjakob/gocryptfs/internal/configfile	0.783s
gocryptfs has been compiled without openssl support but you are still trying to use openssl
FAIL	github.com/rfjakob/gocryptfs/internal/contentenc	0.003s
gocryptfs has been compiled without openssl support but you are still trying to use openssl
FAIL	github.com/rfjakob/gocryptfs/internal/cryptocore	0.003s
ok  	github.com/rfjakob/gocryptfs/internal/ctlsock	0.003s
?   	github.com/rfjakob/gocryptfs/internal/exitcodes	[no test files]
?   	github.com/rfjakob/gocryptfs/internal/fusefrontend	[no test files]
?   	github.com/rfjakob/gocryptfs/internal/fusefrontend_reverse	[no test files]
ok  	github.com/rfjakob/gocryptfs/internal/nametransform	0.002s
?   	github.com/rfjakob/gocryptfs/internal/openfiletable	[no test files]
ok  	github.com/rfjakob/gocryptfs/internal/pathiv	0.003s
ok  	github.com/rfjakob/gocryptfs/internal/prefer_openssl	0.004s
ok  	github.com/rfjakob/gocryptfs/internal/readpassword	0.019s
?   	github.com/rfjakob/gocryptfs/internal/serialize_reads	[no test files]
ok  	github.com/rfjakob/gocryptfs/internal/siv_aead	0.006s
ok  	github.com/rfjakob/gocryptfs/internal/speed	0.007s [no tests to run]
gocryptfs has been compiled without openssl support but you are still trying to use openssl
FAIL	github.com/rfjakob/gocryptfs/internal/stupidgcm	0.002s
?   	github.com/rfjakob/gocryptfs/internal/syscallcompat	[no test files]
?   	github.com/rfjakob/gocryptfs/internal/tlog	[no test files]
ok  	github.com/rfjakob/gocryptfs/tests/cli	0.906s
ok  	github.com/rfjakob/gocryptfs/tests/defaults	0.733s
ok  	github.com/rfjakob/gocryptfs/tests/example_filesystems	2.208s
ok  	github.com/rfjakob/gocryptfs/tests/hkdf_sanity	0.146s
ok  	github.com/rfjakob/gocryptfs/tests/matrix	10.349s
ok  	github.com/rfjakob/gocryptfs/tests/plaintextnames	0.118s
ok  	github.com/rfjakob/gocryptfs/tests/reverse	0.514s
?   	github.com/rfjakob/gocryptfs/tests/test_helpers	[no test files]