sh: Add tests where "export" does not parse differently.

It is planned to expand variable assignments as assignments (no word
splitting, different tilde expansion) when they follow a "declaration
utility" (export, readonly or local). However, a quoted character cannot be
part of a "name" so things like \v=~ are not assignments, and the existing
behaviour applies.

tools/regression/bin/sh/expansion/export1.0

@@ -0,0 +1,13 @@
+# $FreeBSD$
+
+w='@ vv=6'
+
+v=0 vv=0
+export \v=$w
+[ "$v" = "@" ] || echo "Expected @ got $v"
+[ "$vv" = "6" ] || echo "Expected 6 got $vv"
+
+HOME=/known/value
+
+export \v=~
+[ "$v" = \~ ] || echo "Expected ~ got $v"