Kotlin-ize some more tests

[jamesonwilliams]

Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• MacBook Pro M3 (test suite)

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

---

Description

Converting a few more unit tests to Kotlin:

app/src/test/java/org/thoughtcrime/securesms/recipients/RecipientExporterTest.java
app/src/test/java/org/thoughtcrime/securesms/recipients/RecipientUtilTest.java
app/src/test/java/org/thoughtcrime/securesms/util/DelimiterUtilTest.java
core-util/src/test/java/org/signal/core/util/concurrent/LatestPrioritizedSerialExecutorTest.java
libsignal-service/src/test/java/org/whispersystems/signalservice/api/services/DonationsServiceTest.java

Also migrated from Mockito to MocK where appropriate

Verified operation of modified tests:

./gradlew \
    :Signal-Android:testPlayProdDebugUnitTest \
    :core-util:testDebugUnitTest \
    :libsignal-service:test

[greyson-signal]

Thank you very much!