Three tests were guarded by `MBEDTLS_KEY_EXCHANGE_ECJPAKE`,
not `MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED`, as it should be.
Curiously, the guard still functioned as intended, perhaps
because `MBEDTLS_KEY_EXCHANGE_ECJPAKE` is a prefix of
`MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED`.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
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