internal/godoc/dochtml: rename template_test.go to legacy_template_test.go

template_test.go is renamed to legacy_template_test.go, since that test
is ensuring the legacy template isn't accidentally changed.

Change-Id: I2d86f3a127ce1b429e463b74c4ac1a93607f4210
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260730
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/godoc/dochtml/template_test.go b/internal/godoc/dochtml/legacy_template_test.go
similarity index 100%
rename from internal/godoc/dochtml/template_test.go
rename to internal/godoc/dochtml/legacy_template_test.go