Don't test runtime behaviour in buildtime tests.

[infinity0]

client/tpm/tpm_test.go and client/system/system_test.go test behaviour about the runtime but are unit tests that typically run during build time.

These tests are already present in the main program (e.g. tpmNotPresentMsg, IsSafe, SafeTempDir), so the tests are pointless. In fact I have to disable them in the Debian package because they fail on the build machines, e.g. [1]

[1] https://buildd.debian.org/status/fetch.php?pkg=pond&arch=i386&ver=0.1.1-2&stamp=1444737392