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/usr/lib/mysql-testsuite/collections/README.experimental is in percona-xtradb-cluster-testsuite-5.5 5.5.34-25.9+dfsg-0ubuntu4.

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The actual contents of the file can be viewed below.

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The .experimental files in this directory contain names of test cases that
are still in development and whose failures should be considered expected,
instead of regressions.

These files are to be used with the --experimental option of 
mysql-test-run.pl. Please look at its help screen for usage information.

The syntax is as follows:

1) One line per test case.

2) Empty lines and lines starting with a hash (#) are ignored.

3) If any other line contains a blank followed by a hash (#), the hash 
   and any subsequent characters are ignored.

4) The full test case name including the suite and execution mode
   may be specified, for example:
        main.alias 'row'     # bug#00000

4b) Now, combinations will also be covered if only the test name is
    specified, for example:
        rpl.rpl_ps       # Covers 'row', 'mix' and 'stmt'

5) As an exception to item 4, the last character of the test case
   specification may be an asterisk (*). In that case, all test cases that
   start with the same characters up to the last letter before the asterisk
   are considered experimental:
        main.a*  # get rid of main.alias, main.alibaba and main.agliolio

6) Optionally, the test case may be followed by one or more platform
   qualifiers beginning with @ or @!. The test will then be considered
   experimental only/except on that platform. Basic OS names as
   reported by $^O in Perl, or 'windows' are supported, this includes
   solaris, linux, windows, aix, darwin, ...  Example:
        main.alias  @aix @windows    # Fails on those