Limit test_throws to julia versions using cat definition in Compat

On 0.7, calls to Compat.cat without dims would not error due to support for the
deprecated syntax. On 1.0, calls without  dims would error with a different type
of exception. I changed the exception type to be consistent across versions, and
only checked for the exception of versions using Compat's definition of cat.

src/Compat.jl

@@ -1831,7 +1831,7 @@ if VERSION < v"0.7.0-DEV.4738"
     Base.squeeze(A; dims=error("squeeze: keyword argument dims not assigned")) = squeeze(A, dims)
 end
 if VERSION < v"0.7.0-DEV.5165" # julia#27163
-    cat(X...; dims = error("cat: keyword argument dims not assigned")) = Base.cat(dims, X...)
+    cat(X...; dims = throw(UndefKeywordError("cat: keyword argument dims not assigned"))) = Base.cat(dims, X...)
 end
 
 if !isdefined(Base, :selectdim) # 0.7.0-DEV.3976

test/runtests.jl

@@ -1655,7 +1655,9 @@ end
 # 0.7.0-DEV.5165
 @test Compat.cat([1, 2], [3, 4, 5], dims = 1) == [1, 2, 3, 4, 5]
 @test Compat.cat([1, 2], [3, 4], dims = 2) == [1 3; 2 4]
-@test_throws ErrorException Compat.cat([1, 2], [3, 4])
+if VERSION < v"0.7.0-DEV.5165"
+    @test_throws UndefKeywordError Compat.cat([1, 2], [3, 4])
+end
 
 # 0.7.0-DEV.3976
 let A = rand(5,5)