Test failure if run with multiple threads

Split off from #86:

$ JULIA_NUM_THREADS=2 julia runtests.jl 
Skipping Base.active_repl
Skipping Base.active_repl_backend
Skipping Base.active_repl
Skipping Base.active_repl_backend
Test Summary: | Pass  Total
Border        |  102    102
[ Info: Two warnings are expected
┌ Warning: Likely overflow or conversion error detected. Consider specifying the output type, e.g., `imfilter(UInt16, img, kernel, ...)`
└ @ ImageFiltering ~/.julia/dev/ImageFiltering/src/imfilter.jl:625
Test Summary: | Pass  Total
1d            |   60     60
┌ Warning: Likely overflow or conversion error detected. Consider specifying the output type, e.g., `imfilter(Int32, img, kernel, ...)`
└ @ ImageFiltering ~/.julia/dev/ImageFiltering/src/imfilter.jl:625
Test Summary: | Pass  Total
2d widening   |    3      3
Test Summary: | Pass  Total
3d            |   18     18
Test Summary: | Pass  Total
tiling        |   10     10
Test Summary: | Pass  Total
FIR/FFT       | 1191   1191

Error thrown in threaded loop on thread 0: Base.DimensionMismatch(msg="requested indices (1:8, 0:9) and kernel indices (Base.Slice(-1:1), Base.Slice(0:0)) do not agree with indices of padded input, (Base.Slice(0:9), Base.Slice(1:8))")Borders (issue #85): Test Failed at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:335
  Expression: imfilter(A, Kernel.gaussian((1, 1), (3, 3)), Fill(0, (1, 0)))
    Expected: DimensionMismatch("requested indices (1:8, 0:9) and kernel indices (Base.Slice(-1:1), Base.Slice(0:0)) do not agree with indices of padded input, (Base.Slice(0:9), Base.Slice(1:8))")
  No exception thrown
Stacktrace:
 [1] macro expansion at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:335 [inlined]
 [2] macro expansion at /usr/local/julia/julia-1.0/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1083 [inlined]
 [3] top-level scope at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:239

Error thrown in threaded loop on thread 0: Base.DimensionMismatch(msg="requested indices (1:8, 0:9) and kernel indices (Base.Slice(-1:1), Base.Slice(0:0)) do not agree with indices of padded input, (Base.Slice(1:8), Base.Slice(0:9))")Borders (issue #85): Test Failed at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:336
  Expression: imfilter(A, Kernel.gaussian((1, 1), (3, 3)), Fill(0, (0, 1)))
    Expected: DimensionMismatch
  No exception thrown
Stacktrace:
 [1] macro expansion at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:336 [inlined]
 [2] macro expansion at /usr/local/julia/julia-1.0/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1083 [inlined]
 [3] top-level scope at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:239

Error thrown in threaded loop on thread 0: Base.DimensionMismatch(msg="requested indices (1:8, 0:9) and kernel indices (Base.Slice(-1:1), Base.Slice(0:0)) do not agree with indices of padded input, (Base.Slice(1:8), Base.Slice(1:8))")Borders (issue #85): Test Failed at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:337
  Expression: imfilter(A, Kernel.gaussian((1, 1), (3, 3)), Fill(0, (0, 0)))
    Expected: DimensionMismatch
  No exception thrown
Stacktrace:
 [1] macro expansion at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:337 [inlined]
 [2] macro expansion at /usr/local/julia/julia-1.0/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1083 [inlined]
 [3] top-level scope at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:239
Test Summary:       | Pass  Fail  Total
Borders (issue #85) |   35     3     38
ERROR: LoadError: LoadError: Some tests did not pass: 35 passed, 3 failed, 0 errored, 0 broken.
in expression starting at /home/tim/.julia/dev/ImageFiltering/test/2d.jl:238
in expression starting at /home/tim/.julia/dev/ImageFiltering/test/runtests.jl:12

Run JULIA_NUM_THREADS=2 julia runtests.jl and start with the failing cases in test/2d.jl:335-337, reached through test/runtests.jl. Compare the threaded behavior with the expected DimensionMismatch results and inspect the imfilter call path shown in the failure. Done means the Borders tests pass when running with multiple threads.