Tweak CI check

Author: musm

test/runtests.jl

@@ -1,7 +1,7 @@
 using MATLAB
 using Test
 
-is_ci() = get(ENV, "CI", nothing) == "true"
+is_ci() = lowercase(get(ENV, "CI", "false")) == "true"
 
 if !is_ci() # only test if not CI
 include("engine.jl")