html: ensure <search> tag closes <p> and update tests

This change ensures that the <search> tag correctly closes an open <p> tag when encountered during parsing.

Changes:
- Added <search> to the list of elements that should close an open <p> tag in parse.go.
- Updated the second list in parse.go to ensure consistency.
- Updated html/atom/gen.go, table.go, and table_test.go accordingly.
- Modified parse_test.go to use strings.Builder instead of bytes.Buffer.
- Updated test error messages to follow Go’s conventions.
- Fixed an accidental colon in the comment in parse.go.

Change-Id: I5835da69f6bb0e14c483e55b7ae82915ae958dc1
Reviewed-on: https://go-review.googlesource.com/c/net/+/655457
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: Volentie

html/atom/gen.go

@@ -540,6 +540,7 @@ var attributes = []string{
 	"scope",
 	"scoped",
 	"seamless",
+	"search",
 	"selected",
 	"shape",
 	"size",