io/fs: clean up test helper functions

io/fs: clean up test helper functions

Inline the only use of checkMarks which also allows to drop the
always-true report argument. This also ensures the correct line gets
reported in case of an error.

Also remove the unused markTree function and drop the unused testing.T
argument from makeTree.

Change-Id: I4033d3e5ecd929d08ce03c563aa99444e102d931
Reviewed-on: https://go-review.googlesource.com/c/go/+/451615
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>