tests: shell: don't rely on writable test directory

tests: shell: don't rely on writable test directory

Running shell tests from a virtme-ng instance with ro mapped test dir
hangs due to runaway 'awk' reading from stdin instead of the intended
$tmpfile (variable is empty), so add quotes where needed.

0002relative_0 wants to check relative includes. It tries to create a
temporary file in the current directory, which fails as thats readonly
inside the virtme vm instance.

[ -w ! $foo ... did not catch this due to missing "".
Add quotes and return the skip retval so the test gets flagged as skipped.

0013input_descriptors_included_files_0 and 0020include_chain_0 are
switched to normal tmpfiles, there is nothing in the test that needs
relative includes.

Also, get rid of some error tests for subsequent mktemp calls for
scripts that already called 'set -e'.

Signed-off-by: Florian Westphal <fw@strlen.de>