fix(tabs): prevent double input event on mount, and add additional tests

[tmorehouse]

Description of Pull Request:

Minor updates to b-tab and b-tabs code, and additional test suites.

Also removes the double input emit on mount that would occasionally occur.

Note: this does not address issue #2707

PR checklist:

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe: test suites

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact:

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e.
  fixes #xxxx[,#xxxx], where "xxxx" is the issue number)
• The PR should address only one issue or feature. If adding multiple features or fixing a bug
  and adding a new feature, break them into separate PRs if at all possible.
• PR titles should following the
  Conventional Commits naming convention (i.e.
  "fix(alert): not alerting during SSR render", "docs(badge): Updated pill examples, fix typos",
  "chore: fix typo in docs", etc). This is very important, as the CHANGELOG is generated
  from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and
  event changes)
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (does it affect screen reader users or
  keyboard only users? clickable items should be in the tab index, etc)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a
  suggestion issue first and wait for approval before working on it)

[codecov]

Codecov Report

Merging #2748 into dev will increase coverage by 0.42%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##              dev    #2748      +/-   ##
==========================================
+ Coverage   74.92%   75.34%   +0.42%     
==========================================
  Files         176      176              
  Lines        3198     3212      +14     
  Branches      943      947       +4     
==========================================
+ Hits         2396     2420      +24     
+ Misses        585      577       -8     
+ Partials      217      215       -2

Impacted Files	Coverage Δ
src/components/tabs/tab.js	96.77% <100%> (+12.39%)	⬆️
src/utils/dom.js	76.62% <100%> (+0.3%)	⬆️
src/components/tabs/tabs.js	97.94% <91.3%> (+4.76%)	⬆️

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