prefer abs_srcdir over abs_top_srcdir, ...

prefer abs_srcdir over abs_top_srcdir, ...

so that e.g., tests/* may refer to absolute names without
hard-coding the name of the containing directory (tests/, here).
* tests/check.mk (TESTS_ENVIRONMENT): Define abs_srcdir.
* tests/pr/pr-tests: Use $abs_srcdir/pr not $abs_top_srcdir/tests/pr.
* tests/chmod/c-option: Use $abs_srcdir, not $abs_top_srcdir/tests.
* tests/cp/cp-parents: Likewise.
* tests/mkdir/parents: Likewise.
* tests/mkdir/perm: Likewise.
* tests/mv/acl: Likewise.
* tests/mv/backup-is-src: Likewise.
* tests/mv/hard-link-1: Likewise.
* tests/mv/into-self-2: Likewise.
* tests/mv/leak-fd: Likewise.
* tests/mv/mv-special-1: Likewise.
* tests/mv/part-fail: Likewise.
* tests/mv/part-hardlink: Likewise.
* tests/mv/part-rename: Likewise.
* tests/mv/part-symlink: Likewise.
* tests/mv/partition-perm: Likewise.
* tests/mv/sticky-to-xpart: Likewise.
* tests/mv/to-symlink: Likewise.
* tests/rm/one-file-system: Likewise.
* tests/sample-test: Likewise.