[FIX] Fix typo in the name of cudf-testing-targets.cmake

[trxcllnt]

This fix enables local source builds using the build directory as cudf_ROOT to find cudf::cudftestutil targets again.

[vyasr]

rerun tests

[codecov]

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@@               Coverage Diff               @@
##             branch-21.10    #9190   +/-   ##
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  Coverage                ?   10.77%           
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  Files                   ?      115           
  Lines                   ?    19138           
  Branches                ?        0           
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  Hits                    ?     2062           
  Misses                  ?    17076           
  Partials                ?        0           

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[vyasr]

@gpucibot merge