Fix 1065 to improve code coverage for `with_opt.R` to 100%

[ilyaZar]

Fix #1065

Done:

• [x] change filename: {testthat} style suggests "test-with_opt.R" as a file name for tests of functions from "R/with_opt.R" 7f5d6d2a19ff5f9ccf079e985e7df1c845c74c4a

• [x] improve tests for maintenance mode to see whether it works with setting options in a fresh golem 7f5d6d2a19ff5f9ccf079e985e7df1c845c74c4a

• [x] add tests for SHINY_PORT 7f5d6d2a19ff5f9ccf079e985e7df1c845c74c4a and 5d09b498e63051d883847b74c32b64a5b89858f0

• [x] get_golem_options is tested by temporarily altering shiny:::.globals variable 1b833327d016d0196b8b2ad715b9da82cd9a19ce

• [x] corner case with the print=TRUE-flag in with_golem_options() is tested by temporarily overwriting S3-method shiny:::print.shiny.appobj() 5d09b498e63051d883847b74c32b64a5b89858f0

To-Do / Note keeping:

Full coverage reached for file R/with_opt.R:

[VincentGuyader]

we'll be watching all the submissions very closely, thanks a lot!

[ilyaZar]

thanks @VincentGuyader