fix flaky TSVB test

[dmlemeshko]

Summary

There is a test that constantly fails locally and somehow more stable on CI.

The issue is failing click on dropdown option on TSVB Series Option tab, it fails with StaleElementException.

           └- ✖ fail: "visualize app  visual builder Time Series should show the correct count in the legend with "Human readable" duration formatter"
           │      StaleElementReferenceError: stale element reference: element is not attached to the page document
           │   (Session info: chrome=76.0.3809.100)
           │   (Driver info: chromedriver=76.0.3809.68 (420c9498db8ce8fcd190a954d51297672c1515d5-refs/branch-heads/3809@{#864}),platform=Mac OS X 10.14.6 x86_64)

The only working option for me was to click with a mouse (works on both Chrome and Firefox)

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[dmlemeshko]

retest

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[maryia-lapata]

LGTM,
I run tests locally (Mac)

[LeeDr]

LGTM - I ran all 'visual builder' tests locally on Windows with Chrome browser and all 26 tests passed. I made one comment about finding data-test-subj's that start with the value parameter but it's not really a change in this PR.

Just for fun I re-ran the 'visual builder' tests with this change (= instead of ^=) and they all passed;

diff --git a/test/functional/services/combo_box.ts b/test/functional/services/combo_box.ts
index 2e44e0248c..65270eac98 100644
--- a/test/functional/services/combo_box.ts
+++ b/test/functional/services/combo_box.ts
@@ -74,7 +74,7 @@ export function ComboBoxProvider({ getService, getPageObjects }: FtrProviderCont

       if (value !== undefined) {
         const selectOptions = await find.allByCssSelector(
-          `.euiFilterSelectItem[title^="${value.toString().trim()}"]`,
+          `.euiFilterSelectItem[title="${value.toString().trim()}"]`,
           WAIT_FOR_EXISTS_TIME
         );

But I didn't run all tests so not fully tested.

[LeeDr]

I guess this isn't a change in this PR, but I don't understand why we're only looking for the case where the element's title attribute starts with value. I would think we would want to find the element who's title equals value? Maybe this was an issue with data-test-subj's with spaces in them? If that issue is fixed maybe we could improve this here for exact match?

[dmlemeshko]

It is used to match drop-down option that starts with provided text. I remember testing related PR and may confirm we need it for some tests