Improve analysis of racy testcases

This is an initial attempt to introduce some mechanisms to identify
racy testcases present in our testsuite.  As can be seen in previous
discussions, racy tests are really bothersome and cause our BuildBot
to pollute the gdb-testers mailing list with hundreds of
false-positives messages every month.  Hopefully, identifying these
racy tests in advance (and automatically) will contribute to the
reduction of noise traffic to gdb-testers, maybe to the point where we
will be able to send the failure messages directly to the authors of
the commits.

I spent some time trying to decide the best way to tackle this
problem, and decided that there is no silver bullet.  Racy tests are
tricky and it is difficult to catch them, so the best solution I could
find (for now?) is to run our testsuite a number of times in a row,
and then compare the results (i.e., the gdb.sum files generated during
each run).  The more times you run the tests, the more racy tests you
are likely to detect (at the expense of waiting longer and longer).
You can also run the tests in parallel, which makes things faster (and
contribute to catching more racy tests, because your machine will have
less resources for each test and some of them are likely to fail when
this happens).  I did some tests in my machine (8-core i7, 16GB RAM),
and running the whole GDB testsuite 5 times using -j6 took 23 minutes.
Not bad.

In order to run the racy test machinery, you need to specify the
RACY_ITER environment variable.  You will assign a number to this
variable, which represents the number of times you want to run the
tests.  So, for example, if you want to run the whole testsuite 3
times in parallel (using 2 cores), you will do:

  make check RACY_ITER=3 -j2

It is also possible to use the TESTS variable and specify which tests
you want to run:

  make check TEST='gdb.base/default.exp' RACY_ITER=3 -j2

And so on.  The output files will be put at the directory
gdb/testsuite/racy_outputs/.

After make invokes the necessary rules to run the tests, it finally
runs a Python script that will analyze the resulting gdb.sum files.
This Python script will read each file, and construct a series of sets
based on the results of the tests (one set for FAIL's, one for
PASS'es, one for KFAIL's, etc.).  It will then do some set operations
and come up with a list of unique, sorted testcases that are racy.
The algorithm behind this is:

  for state in PASS, FAIL, XFAIL, XPASS...; do
    if a test's state in every sumfile is $state; then
      it is not racy
    else
      it is racy

(The algorithm is actually a bit more complex than that, because it
takes into account other things in order to decide whether the test
should be ignored or not).

IOW, a test must have the same state in every sumfile.

After processing everything, the script prints the racy tests it could
identify on stdout.  I am redirecting this to a file named racy.sum.

Something else that I wasn't sure how to deal with was non-unique
messages in our testsuite.  I decided to do the same thing I do in our
BuildBot: include a unique identifier in the end of message, like:

  gdb.base/xyz.exp: non-unique message
  gdb.base/xyz.exp: non-unique message <<2>>

This means that you will have to be careful about them when you use
the racy.sum file.

I ran the script several times here, and it did a good job catching
some well-known racy tests.  Overall, I am satisfied with this
approach and I think it will be helpful to have it upstream'ed.  I
also intend to extend our BuildBot and create new, specialized
builders that will be responsible for detecting the racy tests every X
number of days.

2016-03-05  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (DEFAULT_RACY_ITER): New variable.
	(CHECK_TARGET_TMP): Likewise.
	(check-single-racy): New rule.
	(check-parallel-racy): Likewise.
	(TEST_TARGETS): Adjust rule to account for RACY_ITER.
	(do-check-parallel-racy): New rule.
	(check-racy/%.exp): Likewise.
	* README (Racy testcases): New section.
	* analyze-racy-logs.py: New file.

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