util subpackage: fix cranky unit test
util subpackage: fix cranky unit test

The new TestTerminalWidth recently added in 9d64b43 fails outside of CI when
multiple packages are tested at once, e.g. a main (base dir) invocation like:

    go test -v -p 1 -timeout 60m -coverpkg=./... ./...

It seems `go test` inherently redirects STDOUT in this situation, whereas
invoking `go test` directly on the util package does not. This commit fixes
the test to be less picky outside of CI environments.

skeema/skeema