[R-Source] Improve unit tests for model

Follow-up-to: 7b3b8ad05545a6cf7c894ccaac3e57bd9ede3716
diff --git a/r/org.eclipse.statet.r.core-tests/src/org/eclipse/statet/internal/r/core/rmodel/SourceModelTests.java b/r/org.eclipse.statet.r.core-tests/src/org/eclipse/statet/internal/r/core/rmodel/SourceModelTests.java
index 16b0be5..d32d264 100644
--- a/r/org.eclipse.statet.r.core-tests/src/org/eclipse/statet/internal/r/core/rmodel/SourceModelTests.java
+++ b/r/org.eclipse.statet.r.core-tests/src/org/eclipse/statet/internal/r/core/rmodel/SourceModelTests.java
@@ -53,12 +53,12 @@
 	
 	
 	public static void assertRegion(final int expectedOffset, final int expectedLength,
-			final @Nullable TextRegion actual, final String prefix) {
+			final @Nullable TextRegion actual, final String elementLabel) {
 		assertNotNull(actual);
 		assertEquals(expectedOffset, actual.getStartOffset(),
-				() -> prefix + ".startOffset" );
+				() -> elementLabel + ".startOffset" );
 		assertEquals(expectedOffset +  expectedLength, actual.getEndOffset(),
-				() -> prefix + ".endOffset" );
+				() -> elementLabel + ".endOffset" );
 	}
 	
 	public static void assertElementName(final int expectedType, final String expectedName,