fix for additional test code via package_coverage(..., code = .)

[bastistician]

package_coverage(..., code = character()) allows for additional test code to be run. The code argument should be a character vector of lines of code. This currently fails for length(code) > 1 as this vector is cat to a test script using sep="", so arrives all in a single line.

A possible fix is attached, also including a test case.
Running this test with current covr before the patch results in the following failure:

── Error (Line 2): package_coverage runs additional test code ─────────────────────
Error: Failure in `/tmp/RtmpTBAKz9/R_LIBS7b74718d8a6a/TestS4-commands.Rout.fail`

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> library('TestS4')
> a(1)a(2)
Error: unexpected symbol in "a(1)a"
Execution halted
Backtrace:
 1. covr::package_coverage(...)
 5. covr:::run_commands(pkg, install_path, code)
 6. covr:::show_failures(dirname(failfile))

[CLAassistant]

All committers have signed the CLA.

[jimhester]

Sorry for the delay in reviewing this, I think what you did looks good, thanks!

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).