Contrib > fix sort button label

[blackbaud-conorwright]

Work done by @Blackbaud-JackMcElhinney

Title was not being set on button with attr prefix. Passed the label as well and created test case to make sure both were set properly.

Resolves: #1191, resolves #1192, resolves #1193

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8670b15). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #1915   +/-   ##
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  Coverage          ?    100%           
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  Files             ?     415           
  Lines             ?    8746           
  Branches          ?    1292           
========================================
  Hits              ?    8746           
  Misses            ?       0           
  Partials          ?       0

Impacted Files	Coverage Δ
src/modules/dropdown/dropdown-menu.component.ts	100% <100%> (ø)
src/modules/sort/sort.component.ts	100% <100%> (ø)

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[blackbaud-johnly]

Added #1916 to document new ARIA property.

[Blackbaud-AlexKingman]

LGTM.