fix setenv and add unit tests

Fixes #3937

tests/setenv.in

@@ -0,0 +1,20 @@
+# Verify the correct behavior of the `setenv` compatibility shim.
+
+# No args to `setenv` should emit the current set of env vars. The first two
+# commands verify that `setenv` does not report non-env vars.
+set -g setenv1 abc
+setenv | grep '^setenv1=$'
+set -gx setenv1 xyz
+setenv | grep '^setenv1=xyz$'
+
+# A single arg should set and export the named var to nothing.
+setenv setenv2
+env | grep '^setenv2=$'
+
+# Three or more args should be an error.
+echo too many arguments test >&2
+setenv var hello you
+
+# Two args should set the named var to the second arg
+setenv setenv3 'hello you'
+setenv | grep '^setenv3=hello you'