chore: don't run solver integration tests in parallel

Hopefully makes metric-based tests less flaky.

Author: triceo

core/src/test/java/ai/timefold/solver/core/config/solver/EnvironmentModeTest.java

@@ -38,12 +38,9 @@
 import ai.timefold.solver.core.testutil.PlannerTestUtils;
 
 import org.junit.jupiter.api.BeforeAll;
-import org.junit.jupiter.api.parallel.Execution;
-import org.junit.jupiter.api.parallel.ExecutionMode;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.EnumSource;
 
-@Execution(ExecutionMode.CONCURRENT)
 class EnvironmentModeTest {
 
     private static final int NUMBER_OF_RANDOM_NUMBERS_GENERATED = 1000;

core/src/test/java/ai/timefold/solver/core/impl/solver/AllowsUnassignedValuesListVariableSolverTest.java

@@ -23,12 +23,9 @@
 import ai.timefold.solver.core.testdomain.list.unassignedvar.TestdataAllowsUnassignedValuesListValue;
 
 import org.assertj.core.api.Assertions;
-import org.junit.jupiter.api.parallel.Execution;
-import org.junit.jupiter.api.parallel.ExecutionMode;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.EnumSource;
 
-@Execution(ExecutionMode.CONCURRENT)
 class AllowsUnassignedValuesListVariableSolverTest {
 
     @ParameterizedTest