KVM: selftests: Refactor stable TSC check to use TEST_REQUIRE()

Refactor the nested TSC scaling test's check on a stable system TSC to
use TEST_REQUIRE() to do the heavy lifting when the system doesn't have
a stable TSC.  Using a helper+TEST_REQUIRE() eliminates the need for
gotos and a custom message.

Cc: Hao Ge <gehao@kylinos.cn>
Cc: Vipin Sharma <vipinsh@google.com>
Link: https://lore.kernel.org/r/20230406001724.706668-1-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>