fix: reset db before each test

The row IDs are unpredictable on subsequent test runs due to the ID
sequence not getting reset. Add a DB fixture for force-resetting the
database state to a blank slate before each test, to ensure things are
in a predictable state.

src/test/kotlin/fi/oph/kitu/KituApplicationTests.kt

@@ -2,13 +2,14 @@ package fi.oph.kitu
 
 import fi.oph.kitu.oppija.Oppija
 import fi.oph.kitu.oppija.OppijaService
+import fi.oph.kitu.test.DBFixture
 import org.junit.jupiter.api.Test
 import org.springframework.beans.factory.annotation.Autowired
 import org.springframework.boot.test.context.SpringBootTest
 import kotlin.test.assertContentEquals
 
 @SpringBootTest
-class KituKotlinApplicationTests {
+class KituKotlinApplicationTests : DBFixture() {
     @Autowired
     private lateinit var oppijaService: OppijaService
 

src/test/kotlin/fi/oph/kitu/test/DBFixture.kt

@@ -0,0 +1,15 @@
+package fi.oph.kitu.test
+
+import org.junit.jupiter.api.BeforeEach
+import org.springframework.beans.factory.annotation.Autowired
+import org.springframework.jdbc.core.JdbcTemplate
+
+abstract class DBFixture {
+    @Autowired
+    private lateinit var jdbcTemplate: JdbcTemplate
+
+    @BeforeEach
+    fun nukeDB() {
+        jdbcTemplate.execute("TRUNCATE oppija RESTART IDENTITY ")
+    }
+}