t9811: replace 'test -f' and '! test -f' with 'test_path_*'

t9811: replace 'test -f' and '! test -f' with 'test_path_*'

Replace the basic shell commands 'test -f', with more modern test
helpers 'test_path_is_file' and 'test_path_is_missing'.
These modern helpers emit useful information when the corresponding
tests fail, unlike 'test -f' and '! test -f'.

The occurrences of '! test -f filename' were replaced by
'file_path_is_missing filename', a stronger guarantee equivalent to
'! test -e filename'.

Co-authored-by: Vinicius Lira de Freitas <redacted>
Signed-off-by: Vinicius Lira de Freitas <redacted>
Signed-off-by: Marcelo Machado Lage <redacted>
Signed-off-by: Junio C Hamano <redacted>