syntax: ensure that |& errors on posix via a test

We were sort of testing it indirectly. Add an obvious test and comment,
since the code threw me off for a moment.

syntax/parser.go

@@ -1855,6 +1855,8 @@ func (p *Parser) gotStmtPipe(s *Stmt) *Stmt {
 	switch p.tok {
 	case orAnd:
 		if p.lang == LangMirBSDKorn {
+			// No need to check for LangPOSIX, as on that language
+			// we parse |& as two tokens.
 			break
 		}
 		fallthrough

syntax/parser_test.go

@@ -1265,6 +1265,10 @@ var shellTests = []errorCase{
 		in:   "|& a",
 		bsmk: `1:1: |& is not a valid start for a statement`,
 	},
+	{
+		in:    "foo |& bar",
+		posix: `1:5: | must be followed by a statement`,
+	},
 	{
 		in:   "let",
 		bsmk: `1:1: "let" must be followed by an expression`,