Add string malloc pressure benchmark #1944

Job	Run time
benchmark-ractor-only (head)	38s
benchmark-ractor (head)	6m 47s
test (head)	42s
benchmark-graph	37s
benchmark-run-once	1m 37s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	13m 25s
benchmark-default (ruby)	12m 0s
benchmark-default (head)	10m 36s
test (ruby)	44s
test (truffleruby)	1m 53s
all-tests	3s
	49m 2s

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