interp: support parentheses in classic test commands

This mimics the logic in the bash test expression parser
in the syntax package, which supported them for years.

Fixes #1036.

interp/interp_test.go

@@ -1611,6 +1611,8 @@ var runTests = []runTest{
 	{"set -o pipefail; [[ -o pipefail ]]", ""},
 	// TODO: we don't implement precedence of && over ||.
 	// {"[[ a == x && b == x || c == c ]]", ""},
+	{"[[ (a == x && b == x) || c == c ]]", ""},
+	{"[[ a == x && (b == x || c == c) ]]", "exit status 1"},
 
 	// classic test
 	{
@@ -1710,6 +1712,8 @@ var runTests = []runTest{
 	{"[ abc != ab* ]", ""},
 	// TODO: we don't implement precedence of -a over -o.
 	// {"[ a = x -a b = x -o c = c ]", ""},
+	{`[ \( a = x -a b = x \) -o c = c ]`, ""},
+	{`[ a = x -a \( b = x -o c = c \) ]`, "exit status 1"},
 
 	// arithm
 	{

interp/test_classic.go

@@ -51,7 +51,7 @@ func (p *testParser) classicTest(fval string, pastAndOr bool) syntax.TestExpr {
 	} else {
 		left = p.classicTest(fval, true)
 	}
-	if left == nil || p.eof {
+	if left == nil || p.eof || p.val == ")" {
 		return left
 	}
 	opStr := p.val
@@ -76,7 +76,7 @@ func (p *testParser) classicTest(fval string, pastAndOr bool) syntax.TestExpr {
 }
 
 func (p *testParser) testExprBase(fval string) syntax.TestExpr {
-	if p.eof {
+	if p.eof || p.val == ")" {
 		return nil
 	}
 	op := testUnaryOp(p.val)
@@ -86,6 +86,15 @@ func (p *testParser) testExprBase(fval string) syntax.TestExpr {
 		p.next()
 		u.X = p.classicTest(op.String(), false)
 		return u
+	case syntax.TsParen:
+		pe := &syntax.ParenTest{}
+		p.next()
+		pe.X = p.classicTest(op.String(), false)
+		if p.val != ")" {
+			p.errf("reached %s without matching ( with )", p.val)
+		}
+		p.next()
+		return pe
 	case illegalTok:
 		return p.followWord(fval)
 	default:
@@ -108,6 +117,8 @@ func testUnaryOp(val string) syntax.UnTestOperator {
 	switch val {
 	case "!":
 		return syntax.TsNot
+	case "(":
+		return syntax.TsParen
 	case "-e", "-a":
 		return syntax.TsExists
 	case "-f":

syntax/tokens.go

@@ -312,6 +312,7 @@ const (
 	TsVarSet                                    // -v
 	TsRefVar                                    // -R
 	TsNot     = UnTestOperator(exclMark)        // !
+	TsParen   = UnTestOperator(leftParen)       // (
 )
 
 type BinTestOperator token