fix test isolation

Make sur that PYENV_TEST_DIR is created if mktemp failed
mock python3 usage by updating create_executable function
fix test by adding the right PATH
update path in plugin test
replace for loop with while read
don't use symlink for stub

test/versions.bats

@@ -238,6 +238,7 @@ OUT
 }
 
 @test "non-bare output shows symlink contents" {
+  stub_system_python
   create_version "1.9.0"
   create_alias "link" "1.9.0"