sh: Add some testcases related to subshells.

These failed in earlier attempts to execute more subshells without forking.
The patches are uncommitted.

tools/regression/bin/sh/execution/subshell1.0

@@ -0,0 +1,6 @@
+# $FreeBSD$
+
+(eval "cd /
+v=$(printf %0100000d 1)
+echo \${#v}")
+echo end

tools/regression/bin/sh/execution/subshell1.0.stdout

@@ -0,0 +1,2 @@
+100000
+end

tools/regression/bin/sh/execution/subshell2.0

@@ -0,0 +1,10 @@
+# $FreeBSD$
+
+f() {
+	x=2
+}
+(
+	x=1
+	f
+	[ "$x" = 2 ]
+)

tools/regression/bin/sh/execution/subshell3.0

@@ -0,0 +1,4 @@
+# $FreeBSD$
+
+(false; exit) && exit 3
+exit 0

tools/regression/bin/sh/execution/subshell4.0

@@ -0,0 +1,3 @@
+# $FreeBSD$
+
+(eval "set v=1"; false) && echo bad; :