[dev.simd] simd: move tests to a subdirectory to declutter "simd"

Change-Id: I5bfa97e30eb9739f2cc2f2282e54666f6786d98a
Reviewed-on: https://go-review.googlesource.com/c/go/+/698175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/simd/genfiles.go

@@ -742,17 +742,19 @@ func (from {{.Base}}{{.WxC}}) ToMask() (to Mask{{.WxC}}) {
 }
 `)
 
+const TD = "internal/simd_test/"
+
 func main() {
 	sl := flag.String("sl", "slice_gen_amd64.go", "file name for slice operations")
 	cm := flag.String("cm", "compare_gen_amd64.go", "file name for comparison operations")
 	mm := flag.String("mm", "maskmerge_gen_amd64.go", "file name for mask/merge operations")
 	op := flag.String("op", "other_gen_amd64.go", "file name for other operations")
 	ush := flag.String("ush", "unsafe_helpers.go", "file name for unsafe helpers")
-	bh := flag.String("bh", "binary_helpers_test.go", "file name for binary test helpers")
-	uh := flag.String("uh", "unary_helpers_test.go", "file name for unary test helpers")
-	th := flag.String("th", "ternary_helpers_test.go", "file name for ternary test helpers")
-	ch := flag.String("ch", "compare_helpers_test.go", "file name for compare test helpers")
-	cmh := flag.String("cmh", "comparemasked_helpers_test.go", "file name for compare-masked test helpers")
+	bh := flag.String("bh", TD+"binary_helpers_test.go", "file name for binary test helpers")
+	uh := flag.String("uh", TD+"unary_helpers_test.go", "file name for unary test helpers")
+	th := flag.String("th", TD+"ternary_helpers_test.go", "file name for ternary test helpers")
+	ch := flag.String("ch", TD+"compare_helpers_test.go", "file name for compare test helpers")
+	cmh := flag.String("cmh", TD+"comparemasked_helpers_test.go", "file name for compare-masked test helpers")
 	flag.Parse()
 
 	if *sl != "" {

src/simd/internal/simd_test/no_tag.go

@@ -0,0 +1,10 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package simd
+
+// This file has no build tag, so that go generate can run without a build tag.
+// It does the same thing as go generate in the grandparent directory.
+
+//go:generate go run -C ../.. genfiles.go