Make the NNlib tests more robust

[ChrisRackauckas]

The NNlib tests were found to not be catching many issues because they committed a few sins:

1. The tests leaked between each other, so definitions in one can be effected by the other. This is fixed by using SafeTestsets.
2. The tests did not have good coverage of potential use cases (i.e. no integration testing), and so they missed true failures. This is fixed by at least adding a downstream test on Flux.jl. It should probably hook into any GPU-based tests as well.
3. It used a Manifest when testing, so errors that would be seen on user setups were not caught in the tests.

This should now correctly give red tests.

[ChrisRackauckas]

Locally I see:

(NNlib) pkg> test NNlib
   Testing NNlib
 Resolving package versions...
Test Summary:        | Pass  Total
Activation Functions |  257    257
[ Info: Skipping Convolutional fuzzing tests, set NNLIB_TEST_FUZZING=true to run them
∇depthwiseconv_filter_direct/∇depthwiseconv_data_direct: Test Failed at C:\Users\accou\.julia\dev\NNlib\test\conv.jl:540
  Expression: ddims(∇conv_filter(x, dy, cdims)) == dw_pad
   Evaluated: [NaN, 150.0] == [135.0, 150.0]
Stacktrace:
 [1] top-level scope at C:\Users\accou\.julia\dev\NNlib\test\conv.jl:540
 [2] top-level scope at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.3\Test\src\Test.jl:1107
 [3] top-level scope at C:\Users\accou\.julia\dev\NNlib\test\conv.jl:513
 [4] top-level scope at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.3\Test\src\Test.jl:1107
 [5] top-level scope at C:\Users\accou\.julia\dev\NNlib\test\conv.jl:449
 [6] top-level scope at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.3\Test\src\Test.jl:1107
 [7] top-level scope at C:\Users\accou\.julia\dev\NNlib\test\conv.jl:446
[ Info: Skipping Depthwise Convolutional fuzzing tests, set NNLIB_TEST_FUZZING=true to run them
 58.176720 seconds (143.73 M allocations: 7.287 GiB, 3.04% gc time)
   Testing NNlib tests passed

The testsets in the conv part seem to be nested in such a way that when they fail they don't trigger a failure?

[ChrisRackauckas]

The current test failure traces back to:

https://travis-ci.org/FluxML/NNlib.jl/jobs/619051416#L203-L238

meaning that #142 is the culprit. Before that, the library had other hidden test failures:

https://travis-ci.org/FluxML/NNlib.jl/jobs/612356942#L207-L259

So I think the current issues may trace back to that PR? @staticfloat

[ChrisRackauckas]

The downstream tests reproduce the expected failures:

https://travis-ci.org/FluxML/NNlib.jl/jobs/628608690#L488-L517

[ChrisRackauckas]

Also, this sets Travis to -1 instead of -1.1 so it stays up to date!

[DhairyaLGandhi]

I think testing should happen on multiple Julia versions regardless. Adding testing on 1.1 forces us to not break code on earlier versions too

[ChrisRackauckas]

Added tests for older versions