Define environment variable TARTEST_SKIP_LARGE_FILES=1 in order
to skip tests that require lots of disk space.
* tests/testsuite.at (AT_SKIP_LARGE_FILES): New macro.
* tests/sparse03.at: Mark test with AT_SKIP_LARGE_FILES.
* tests/sparse05.at: Likewise.
* tests/star/pax-big-10g.at: Likewise.
* tests/star/ustar-big-2g.at: Likewise.
* tests/star/ustar-big-8g.at: Likewise.