Summary:
When GNU parallel is available, "make check" tests are now run in parallel.
When /dev/shm is usable, we tell those tests to create temporary files therein.
Now, the longest-running single test, db_test, (which is composed of hundreds of sub-tests)
is no longer run sequentially: instead, each of its sub-tests is run independently, and can
be parallelized along with all other tests. To make that process easier, this change
creates a temporary directory, "t/", in which it puts a small script for each of those
subtests. The output from each parallel-run test is now saved in t/log-TEST_NAME.

When GNU parallel is not available, we run the tests in sequence, just as before.
If GNU parallel is available and you don't like the default of running one subtest
per core, you can invoke "make J=1 check" to run only one test at a time.
Beware: this will take a long time, and it starts with the two longest-running tests, so you
will wait for a long time before seeing any results. Instead, if you want to use fewer resources
but still see useful progress, try "make J=60% check". That will attempt to ensure that 60% of
the cores are occupied by test runs.

To watch progress of individual tests (duration, success (PASS-or-FAIL), name), run "make watch-log"
in the same directory from another window.  That will start with something like this:

and when complete should show numbers/names like this:

  Every 0.1s: sort -k7,7nr -k4,4gr LOG|perl -n -e '@a=split("\t",$_,-1); $t=$a[8]; $t =~ s,^\./,,;' -e '$t =~ s, >.*,,; chomp $t;' -e '$t =~ /.*--gtest_filter=...  Wed Apr  1 10:51:42 2015

  152.221 PASS t/DBTest.FileCreationRandomFailure
  109.280 PASS t/DBTest.EncodeDecompressedBlockSizeTest
   82.315 PASS reduce_levels_test
   77.812 PASS t/DBTest.CompactionFilterWithValueChange
   73.236 PASS backupable_db_test
   63.428 PASS deletefile_test
   57.248 PASS table_test
   55.665 PASS prefix_test
   49.816 PASS t/DBTest.RateLimitingTest
  ...

Test Plan:
Timings (measured so as to exclude compile and link times):
With this change, all tests complete in 2m40s on a system for which nproc prints 32.
Prior to this this change, "make check" would take 24.5 minutes on that same system.

Here are durations (in seconds) of the longest-running subtests:

152.435 PASS t/DBTest.FileCreationRandomFailure
107.070 PASS t/DBTest.EncodeDecompressedBlockSizeTest
 81.391 PASS ./reduce_levels_test
 71.587 PASS ./backupable_db_test
 61.746 PASS ./deletefile_test
 57.960 PASS ./table_test
 55.230 PASS ./prefix_test
 54.060 PASS t/DBTest.CompactionFilterWithValueChange
 48.873 PASS t/DBTest.RateLimitingTest
 47.569 PASS ./fault_injection_test
 46.593 PASS t/DBTest.Randomized
 42.662 PASS t/DBTest.CompactionFilter
 31.793 PASS t/DBTest.SparseMerge
 30.612 PASS t/DBTest.CompactionFilterV2
 25.891 PASS t/DBTest.GroupCommitTest
 23.863 PASS t/DBTest.DynamicLevelMaxBytesBase
 22.976 PASS ./rate_limiter_test
 18.942 PASS t/DBTest.OptimizeFiltersForHits
 16.851 PASS ./env_test
 15.399 PASS t/DBTest.CompactionFilterV2WithValueChange
 14.827 PASS t/DBTest.CompactionFilterV2NULLPrefix

Reviewers: igor, sdong, rven, yhchiang, igor.sugak

Reviewed By: igor.sugak

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D35379