fix(tabs): fix initial value handling (closes #2656)

[jacobmllr95]

Description of Pull Request:

This PR fixes the initial value (v-model) handling by wrapping the first updateTabs() call in the created() hook in a $nextTick() to make sure that the tabs have been created.

Some initial test for the <b-tabs> component are added to back this up.

Closes #2656.

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:

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 #2661 into dev will increase coverage by 2.07%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##              dev   #2661      +/-   ##
=========================================
+ Coverage   71.92%     74%   +2.07%     
=========================================
  Files         173     173              
  Lines        3199    3201       +2     
  Branches      938     939       +1     
=========================================
+ Hits         2301    2369      +68     
+ Misses        655     610      -45     
+ Partials      243     222      -21

Impacted Files	Coverage Δ
src/components/tabs/tabs.js	93.18% <75%> (+49.33%)	⬆️
src/components/tabs/tab.js	84.37% <0%> (+6.25%)	⬆️

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[tmorehouse]

For this one, I think I might add in a few extra tests for functionality (by including a few b-tab sub components inside a b-tabs). I'll see what I can do this weekend.