fix(reporter): use default error formatter for JUnit

[hi-ogawa]

Description

(Recreated #5626 in my fork's branch)

• closes JUnit reporter: ‘Unknown Error: undefined’ #5603

Since we now have reusable printErrorWrapper (introduced for github actions reporter #5093) to generate the default error formatting, probably using it for Junit would make sense.

This wrapper supports all the good things automatically:

• expected/actual diff
• stack
• formatting of throw {somePlainObject} (which is what OP mentioned)

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[hi-ogawa]

These reporter tests are rather difficult to update since it tries to mock out the entire Vitest instance.
What they tests are mostly only for outputFile option handling, so having something complex doesn't seem to worth it, so I simplified tests in /reporters.spec.ts in favor of integration testing in /junit.test.ts.