dsdb group_audit tests: fix use of strncmp

Replace the uses of:
   strncmp(expected, value, strlen(expected))
With:
    strcmp(expected, value)

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
22 jobs for gary-group-audit-tests in 199 minutes and 20 seconds (queued for 2 seconds)
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