test: fix BOM parsing assertion in parse_with_opts

[xuh0923]

Issue
The UTF-8 BOM parsing test in tests/parse_with_opts.c parsed both BOM and non-BOM inputs, but asserted with_bom twice.

That means the test did not actually verify that the plain {} input parsed successfully, even though the comparison below depends on both items being valid.

Changes

• replace the duplicated with_bom assertion with an assertion for without_bom

Verification

• build and run tests/parse_with_opts.c with gcc -Wall -Wextra -Werror -std=c89