Correctly set the variant bits of uuid1

[GunnarFarneback]

The uuid1 function has been incorrect since it was added to Base in Julia 0.4, in that it fails to set the variant bits specified in section 4.1.1 of RFC 4122. This is fixed in this PR and a test is added.

The other implemented uuid versions are doing this correctly.

[nsajko]

Error in testset UUIDs:
Test Failed at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-P2C257CXGN.0/build/default-honeycrisp-P2C257CXGN-0/julialang/julia-master/julia-8fc4e1cd92/share/julia/stdlib/v1.13/UUIDs/test/runtests.jl:48
  Expression: uuid_version(u1) == 2
   Evaluated: 1 == 2
Error in testset UUIDs:
Test Failed at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-P2C257CXGN.0/build/default-honeycrisp-P2C257CXGN-0/julialang/julia-master/julia-8fc4e1cd92/share/julia/stdlib/v1.13/UUIDs/test/runtests.jl:49
  Expression: uuid_version(u4) == 2
   Evaluated: 4 == 2
Error in testset UUIDs:
Test Failed at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-P2C257CXGN.0/build/default-honeycrisp-P2C257CXGN-0/julialang/julia-master/julia-8fc4e1cd92/share/julia/stdlib/v1.13/UUIDs/test/runtests.jl:50
  Expression: uuid_version(u5) == 2
   Evaluated: 5 == 2
Error in testset UUIDs:
Test Failed at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-P2C257CXGN.0/build/default-honeycrisp-P2C257CXGN-0/julialang/julia-master/julia-8fc4e1cd92/share/julia/stdlib/v1.13/UUIDs/test/runtests.jl:51
  Expression: uuid_version(u7) == 2
   Evaluated: 7 == 2
ERROR: LoadError: Test run finished with errors

[GunnarFarneback]

Copy and paste error, should be fixed now.

[StefanKarpinski]

This looks good to me as-is, but if we wanted to define a uuid_variant function that's a bit more general, we could do this:

"""
    uuid_variant(u::UUID) :: Symbol

Return UUID variant as one of the following symbols:
- `:NCS`
- `:RFC4122`
- `:Microsoft`
- `:Reserved`
"""
function uuid_variant(u::UUID)
    v = UInt8((u.value >> 61) & 0x7)
    v ≤ 0x3 ? :NCS_compat :
    v ≤ 0x5 ? :RFC4122    :
    v ≤ 0x6 ? :Microsoft  :
              :Reserved
end

Asking about the version seems to only make sense if the variant is RFC4122.

[GunnarFarneback]

Better do that in a separate PR since this is a pure bugfix, which can be backported.