Add a @benchmarks_test decorator for test method we want to categorize as benchmarks test.

The test driver now takes an option "+b" which enables to run just the benchmarks tests.
By default, tests decorated with the @benchmarks_test decorator do not get run.

Add an example benchmarks test directory which contains nothing for the time being,
just to demonstrate the @benchmarks_test concept.

For example,

$ ./dotest.py -v benchmarks

...

----------------------------------------------------------------------
Collected 2 tests

1: test_with_gdb (TestRepeatedExprs.RepeatedExprssCase)
   Test repeated expressions with gdb. ... skipped 'benchmarks tests'
2: test_with_lldb (TestRepeatedExprs.RepeatedExprssCase)
   Test repeated expressions with lldb. ... skipped 'benchmarks tests'

----------------------------------------------------------------------
Ran 2 tests in 0.047s

OK (skipped=2)
$ ./dotest.py -v +b benchmarks

...

----------------------------------------------------------------------
Collected 2 tests

1: test_with_gdb (TestRepeatedExprs.RepeatedExprssCase)
   Test repeated expressions with gdb. ... running test_with_gdb
benchmarks result for test_with_gdb
ok
2: test_with_lldb (TestRepeatedExprs.RepeatedExprssCase)
   Test repeated expressions with lldb. ... running test_with_lldb
benchmarks result for test_with_lldb
ok

----------------------------------------------------------------------
Ran 2 tests in 0.270s

OK

Also mark some Python API tests which are missing the @python_api_test decorator.

llvm-svn: 136553