PREVIOUS TO MYSQL 8.0

Description :
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The mysqld--defaults-file test fails when the test suite is run from a
non-canonical path, which happens when the current working directory
when mysql-test-run.pl is started contains a symbolic link.

The problem is that this test case uses --replace-result with
$MYSQL_TEST_DIR. This variable is a potentially non-canonical path
based on the current working directory when mtr is started. However,
the path in the expected error message from mysqld contains a
canonical path. This means it does not contain $MYSQL_TEST_DIR if
mtr's working directory is not the canonical path of the working
directory.

Because other tests produce output that may contain non-canonical
paths, making $MYSQL_TEST_DIR always canonical is not a fix.

Fix :
-----
Introduced a new environment variable '$ABS_MYSQL_TEST_DIR' which will
contin the canonical path to the test directory and replaced
$MYSQL_TEST_DIR with the new variable in main.mysqld--defaults-file
test file.

This is a back-port of BUG#24579973.

Change-Id: I3b8df6f2d7ce2b04e188a896d76250cc1addbbc1