t/t1517: mark tests that fail with GIT_TEST_INSTALLED

t/t1517: mark tests that fail with GIT_TEST_INSTALLED

The changes added by 39fc408562 (t/t1517: automate `git subcmd -h` tests
outside a repository, 2025-08-08) to automatically loop over all "main"
Git commands will, when run against an installed build using
GIT_TEST_INSTALLED rather than the build in the build directory, include
some extra git-gui commands that are installed by `make install`.  These
fail the test, so record them as such.

Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>