Try even harder to suppress pak's output

[jennybc]

Somehow the nested use of callr (first layer is parallel testthat workers, second layer is inside pak) gives rise to some very difficult to suppress output. This is just about reducing noise in the testthat report.

[jennybc]

Before this PR:

> test_check("devtools")
  Starting 2 test processes.
  callr_message:
  callr_message: → Will download 1 package with unknown size.
  callr_message: ℹ No downloads are needed
  callr_message: ✔ : [1.9s]
  callr_message:
  callr_message: → Will download 1 package with unknown size.
  callr_message: ℹ No downloads are needed
  callr_message: ✔ : [323ms]
  callr_message:
  callr_message: → Will download 1 package with unknown size.
  callr_message: ℹ No downloads are needed
  callr_message: ✔ : [297ms]
  callr_message:
  callr_message: → Will download 1 package with unknown size.
  callr_message: ℹ No downloads are needed
  callr_message: ✔ : [234ms]
  callr_message:
  callr_message: → Will download 1 package with unknown size.
  callr_message: ℹ No downloads are needed
  callr_message: ✔ : [295ms]
  callr_message:
  callr_message: → Will download 1 package with unknown size.
  callr_message: ℹ No downloads are needed
  callr_message: ✔ : [271ms]
  callr_message:
  callr_message: → Will download 1 package with unknown size.
  callr_message: ℹ No downloads are needed
  callr_message: ✔ : [380ms]
  [ FAIL 0 | WARN 0 | SKIP 7 | PASS 133 ]

After this PR:

> test_check("devtools")
  Starting 2 test processes.
  [ FAIL 0 | WARN 0 | SKIP 7 | PASS 133 ]