test: bump test-runner, improve coverage

[web-padawan]

Updated the tests to improve coverage after trying mutation tests. See a report for remaining (not 100% covered) tests below.

Stryker report

#27. [Survived] BooleanLiteral
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:76:67
-         this.addEventListener('focus', () => this._setFocused(true), true);
+         this.addEventListener('focus', () => this._setFocused(true), false);

Ran all tests for this mutant.
#31. [Survived] BooleanLiteral
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:77:67
-         this.addEventListener('blur', () => this._setFocused(false), true);
+         this.addEventListener('blur', () => this._setFocused(false), false);

Ran all tests for this mutant.
#37. [Survived] StringLiteral
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:82:39
-             document.removeEventListener('mouseup', mouseUpListener);
+             document.removeEventListener("", mouseUpListener);

Ran all tests for this mutant.
#44. [Survived] ConditionalExpression
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:97:10
-         if (this.hasAttribute('active')) {
+         if (true) {

Ran all tests for this mutant.
#65. [Survived] StringLiteral
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:124:37
-           this.setAttribute('focused', '');
+           this.setAttribute('focused', "Stryker was here!");

Ran all tests for this mutant.
#71. [Survived] StringLiteral
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:126:42
-             this.setAttribute('focus-ring', '');
+             this.setAttribute('focus-ring', "Stryker was here!");

Ran all tests for this mutant.
#81. [Survived] StringLiteral
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:141:36
-           this.setAttribute('active', '');
+           this.setAttribute('active', "Stryker was here!");

Ran all tests for this mutant.
#88. [Survived] Regex
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:152:10
-         if (/^( |SpaceBar|Enter)$/.test(event.key) && !event.defaultPrevented) {
+         if (/( |SpaceBar|Enter)$/.test(event.key) && !event.defaultPrevented) {

Ran all tests for this mutant.
#89. [Survived] Regex
/Users/sergey/vaadin/vaadin-item/src/vaadin-item-mixin.js:152:10
-         if (/^( |SpaceBar|Enter)$/.test(event.key) && !event.defaultPrevented) {
+         if (/^( |SpaceBar|Enter)/.test(event.key) && !event.defaultPrevented) {