Fix @has checks "no closing quotation" error

Apparently `"foo\""` has different behavior from `'foo\''` in Python
shlex. See the [discussion on Zulip][z] for more.

[z]: https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/.40has.20checks.20.22no.20closing.20quotation.22

src/test/rustdoc/smart-punct.rs

@@ -25,6 +25,6 @@
 // @has "foo/index.html" "//p" "This is the “start” of the ‘document’! How’d you know that “it’s” the start?"
 // @has "foo/index.html" "//h1" "Header with “smart punct’”"
 // @has "foo/index.html" '//a[@href="https://www.rust-lang.org"]' "link with “smart punct’” – yessiree!"
-// @has "foo/index.html" '//code' 'this inline code -- it shouldn\'t have "smart punct"'
-// @has "foo/index.html" '//pre' 'let x = "don\'t smart-punct me -- please!";'
-// @has "foo/index.html" '//pre' 'I say "don\'t smart-punct me -- please!"'
+// @has "foo/index.html" '//code' "this inline code -- it shouldn't have \"smart punct\""
+// @has "foo/index.html" '//pre' "let x = \"don't smart-punct me -- please!\";"
+// @has "foo/index.html" '//pre' "I say \"don't smart-punct me -- please!\""