sh: Add tests for modifying an alias (r242766).

Note: parser/alias10.0 will eat a lot of memory/cpu time when it fails (with
the old sh).

tools/regression/bin/sh/builtins/alias3.0

@@ -0,0 +1,12 @@
+# $FreeBSD$
+set -e
+
+unalias -a
+alias foo=bar
+alias bar=
+alias quux="1 2 3"
+alias foo=bar
+alias bar=
+alias quux="1 2 3"
+alias
+alias foo

tools/regression/bin/sh/builtins/alias3.0.stdout

@@ -0,0 +1,4 @@
+bar=''
+foo=bar
+quux='1 2 3'
+foo=bar

tools/regression/bin/sh/parser/alias10.0

@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+alias echo='echo'
+alias echo='echo'
+[ "`eval echo b`" = b ]