efi_selftest: fix variables test for GetNextVariableName()

There is a bug in efi variables test.
Fix it with some cosmetic improvements.

Please note that efi variables test still fails at QueryVariableInfo()
and GetVariable(), but this is not due to a change in this patch.
  ==8<==
  Testing EFI API implementation

  Selected test: 'variables'

  Setting up 'variables'
  Setting up 'variables' succeeded

  Executing 'variables'
  .../u-boot/lib/efi_selftest/efi_selftest_variables.c(60):
  TODO: QueryVariableInfo failed
  .../u-boot/lib/efi_selftest/efi_selftest_variables.c(131):
  TODO: GetVariable returned wrong length 7
  .../u-boot/lib/efi_selftest/efi_selftest_variables.c(133):
  TODO: GetVariable returned wrong value
  Executing 'variables' succeeded

  Boot services terminated

  Summary: 0 failures
  ==>8==

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>