syntax: backquotes affect backslashes in single quotes too

I incorrectly understood that backquote command substitutions
did not affect backslashes in single quotes. For example,

    $ echo `echo "1.2" | sed -e 's/\./\\\\./g'`
    1\.2

would be formatted by shfmt as the following, with different output:

    $ echo $(echo "1.2" | sed -e 's/\./\\\\./g')
    1\\.2

when in fact we do want to deduplicate the backslashes:

    $ echo $(echo "1.2" | sed -e 's/\./\\./g')
    1\.2

Tweak the existing test which expected the wrong result.
Thankfully, we simplify the code now with the fix.

Fixes #1041.

syntax/filetests_test.go

@@ -1797,7 +1797,7 @@ var fileTests = []testCase{
 	{
 		Strs: []string{
 			`$(echo '\' 'a\b' "\\" "a\a")`,
-			"`" + `echo '\' 'a\b' "\\\\" "a\a"` + "`",
+			"`" + `echo '\' 'a\\b' "\\\\" "a\a"` + "`",
 		},
 		common: cmdSubst(stmt(call(
 			litWord("echo"),
@@ -4589,7 +4589,7 @@ func recursiveSanityCheck(tb testing.TB, src string, v any) {
 				return
 			}
 		}
-		tb.Errorf("Expected one of %q at %d in %q, found %q",
+		tb.Errorf("Expected one of %q at %s in %q, found %q",
 			strs, pos, src, gotErr)
 	}
 	checkNodePosEnd := func(n Node) {
@@ -4701,7 +4701,7 @@ func recursiveSanityCheck(tb testing.TB, src string, v any) {
 			tb.Errorf("Lit without newlines has Pos/End lines %d and %d",
 				posLine, endLine)
 		case strings.Contains(src, "`") && strings.Contains(src, "\\"):
-			// removed quotes inside backquote cmd substs
+			// removed backslashes inside backquote cmd substs
 			val = ""
 		case end < len(src) && (src[end] == '\n' || src[end] == '`'):
 			// heredoc literals that end with the
@@ -4794,7 +4794,12 @@ func recursiveSanityCheck(tb testing.TB, src string, v any) {
 		if x.Dollar {
 			valuePos = posAddCol(valuePos, 1)
 		}
-		checkPos(valuePos, x.Value)
+		val := x.Value
+		if strings.Contains(src, "`") && strings.Contains(src, "\\") {
+			// removed backslashes inside backquote cmd substs
+			val = ""
+		}
+		checkPos(valuePos, val)
 		if x.Dollar {
 			checkPos(x.Left, "$'")
 		} else {

syntax/lexer.go

@@ -412,11 +412,6 @@ func (p *Parser) peekByte(b byte) bool {
 func (p *Parser) regToken(r rune) token {
 	switch r {
 	case '\'':
-		if p.openBquotes > 0 {
-			// bury openBquotes
-			p.buriedBquotes = p.openBquotes
-			p.openBquotes = 0
-		}
 		p.rune()
 		return sglQuote
 	case '"':

syntax/parser.go

@@ -389,9 +389,6 @@ type Parser struct {
 
 	// lastBquoteEsc is how many times the last backquote token was escaped
 	lastBquoteEsc int
-	// buriedBquotes is like openBquotes, but saved for when the parser
-	// comes out of single quotes
-	buriedBquotes int
 
 	rxOpenParens int
 	rxFirstPart  bool
@@ -435,7 +432,7 @@ func (p *Parser) reset() {
 	p.openStmts = 0
 	p.heredocs, p.buriedHdocs = p.heredocs[:0], 0
 	p.parsingDoc = false
-	p.openBquotes, p.buriedBquotes = 0, 0
+	p.openBquotes = 0
 	p.accComs, p.curComs = nil, &p.accComs
 	p.litBatch = nil
 	p.wordBatch = nil
@@ -1124,10 +1121,6 @@ func (p *Parser) wordPart() WordPart {
 				sq.Right = p.nextPos()
 				sq.Value = p.endLit()
 
-				// restore openBquotes
-				p.openBquotes = p.buriedBquotes
-				p.buriedBquotes = 0
-
 				p.rune()
 				p.next()
 				return sq