Fix truncate benchmarks, add more benchmark tests for longer strings

[Nigel2392]

The benchmark tests for strings.(Namespace).Truncate was rather simple, only testing for very short strings.

I have added 4 benchmarks, one for long and medium plain text and one for long and medium HTML text.

Results (from my local machine) can be seen in the following table:

goos: windows
goarch: amd64
pkg: github.com/gohugoio/hugo/tpl/strings
cpu: 12th Gen Intel(R) Core(TM) i5-1235U
BenchmarkTruncate/Plain_text_(medium)-12         	214671	       5737 ns/op	      384 B/op	       1 allocs/op
BenchmarkTruncate/With_HTML_(medium)-12          	192727	       6150 ns/op	    1248 B/op	       3 allocs/op
BenchmarkTruncate/Plain_text_(long)-12           	          40849	     27711 ns/op	    2048 B/op	       1 allocs/op
BenchmarkTruncate/HTML_text_(long)-12            	          40664	     30509 ns/op	    6144 B/op	       3 allocs/op
PASS
ok  	github.com/gohugoio/hugo/tpl/strings	9.075s

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[Nigel2392]

The benchmark tests are all wrong, even previously. The strings should have been cast to template.HTML.
Will fix tomorrow.

[Nigel2392]

The new benchmark results:

BenchmarkTruncate/Plain_text-12         	 6716235	       182.9 ns/op	      16 B/op	       1 allocs/op
BenchmarkTruncate/With_link-12          	  983589	      1286 ns/op	     161 B/op	       5 allocs/op
BenchmarkTruncate/Plain_text_(medium)-12         	  204534	      5946 ns/op	     384 B/op	       1 allocs/op
BenchmarkTruncate/With_HTML_(medium)-12          	   45519	     25717 ns/op	    2003 B/op	      24 allocs/op
BenchmarkTruncate/Plain_text_(long)-12           	   41742	     28162 ns/op	    2048 B/op	       1 allocs/op
BenchmarkTruncate/HTML_text_(long)-12            	    8776	    144759 ns/op	    4657 B/op	      48 allocs/op