VistATestClient: handle non-default initial cache prompt

Change waitForPrompt to expect('>') instead.

OSEHRA-Id: http://issues.osehra.org/browse/OAT-54

Change-Id: I5161b7274f3797568f711b23c68b5f8428fbbaa5
Merge "VistATestClient: handle non-default initial cache prompt"