[Form] Fixed a method name.

1858b96b7dc473fcab09533b17ace9a24b8d3d86 introduced a mocked context and therefore getExecutionContext() is now called getMockExectionContext().
merged branch jakzal/form-validator-tests-fix (PR #7996)

This PR was merged into the 2.2 branch.

Discussion
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Fixed failing 2.2 tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

1858b96b7dc473fcab09533b17ace9a24b8d3d86 introduced a mocked context and therefore `getExecutionContext()` is was renamed to `getMockExectionContext()`.

Commits
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ef87ba7 [Form] Fixed a method name.