[Tests] `nvm exec`/`nvm run`: add `--silent` tests #276

Job	Run time
tests (install_script, bash)	1m 4s
tests (sourcing, sh)	43s
tests (sourcing, bash)	48s
tests (sourcing, dash)	44s
tests (sourcing, zsh)	1m 9s
tests (slow, sh)	3m 14s
tests (slow, bash)	3m 14s
tests (slow, dash)	3m 12s
tests (slow, zsh)	3m 46s
tests (installation_iojs, sh)	1m 2s
tests (installation_iojs, bash)	1m 7s
tests (installation_iojs, dash)	1m 4s
tests (installation_iojs, zsh)	1m 21s
all test suites, all shells	1s
	22m 29s

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