This PR was merged into the 2.7 branch.

Discussion
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[Form] Hardened form type tests

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

This one the PRs to come targeting 2.7 needed to harden the code base before doing some changes in master.

It takes care of the form types tests part (even if some other tests will be added later, it will also be easier after it), and unlocks merging #21481.
It also back ports tests added in #18357 for the TextType.

Since it's very hard to merge branches when modifying tests because of BC, and making every test type extend the base type test would involve many function calls to get the tested type, the function `getTestedType` is no longer abstract and return a constant to override instead, it's much better for performance, I didn't change the call in the base type test to keep BC but I suggest to deprecate it in master. Even if those are tests I really think it is worth keeping BC here.

The constants also ease testing in the ecosystem of form related libraries that need to be compatible with Symfony 2.7 and 3. I think using "test" as both prefix and suffix on namespaces, classes and names of the constants should discourage using them in real application code. Since this is just about our test suite, I don't think this should be considered a feature, so tis change should be good for 2.7.

Two other PRs will follow to solve conflicts in 2.8 and 3.2.

I missed last month patches, I hope I won't this time :).

Commits
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8cfc3e9 [Form] Hardened form type tests

This PR was merged into the 3.2 branch.

Discussion
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[Form] Fixed empty conversion of Intl types

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20759, #21475
| License       | MIT
| Doc PR        | ~

Commits
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e43bfaf [Form] Fixed empty conversion of Intl types