fix: Flaky TestForDeps tests

[pbitty]

The executor output for parallel task (and dep) execution can come out one character at a time, so even though the tests use SyncWriter, the writer's lock doesn't prevent interleaving.

Using group output forces the task output to be buffered and kept together before being printed.

Fixes #1819.

[pbitty]

With the debug print output:

• Prior to the fix:

% go test -run TestForDeps/loop-explicit -count 1
len: 1
len: 1
len: 1
len: 1
len: 1
len: 1
PASS
ok  	github.com/go-task/task/v3	0.802s

• After the fix:

% go test -run TestForDeps/loop-explicit -count 1
len: 0
len: 2
len: 0
len: 2
len: 0
len: 2
PASS
ok  	github.com/go-task/task/v3	0.590s

The 2-character output of each task gets lumped together into a single Write() call.

[andreynering]

Thanks @pbitty!