Move threaded testsets to main process to reduce Base test noise

[IanButterworth]

Moving the @testset out of the child process here means it won't leak into the printing in Base tests like:

Statistics                               (5) |    95.59 |   3.37 |  3.5 |    6592.22 |  2666.43
      From worker 7:	Test Summary:              | Pass  Total   Time
      From worker 7:	threaded SuiteSparse tests |    1      1  29.8s
SuiteSparse                              (7) |   242.43 |   5.38 |  2.2 |   11184.90 |  2365.01

Noticed while reducing the noise in Base tests JuliaLang/julia#44444

[codecov-commenter]

Codecov Report

Merging #63 (b33eab5) into master (f63732c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #63   +/-   ##
=======================================
  Coverage   89.26%   89.27%           
=======================================
  Files           5        5           
  Lines        1360     1361    +1     
=======================================
+ Hits         1214     1215    +1     
  Misses        146      146           

Impacted Files	Coverage Δ
src/umfpack.jl	86.43% <0.00%> (+0.03%)	⬆️

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