Add test for colour outputs

Test two error conditions that cover all current colour outputs.

pkl-core/src/test/kotlin/org/pkl/core/ErrorColoringTest.kt

@@ -0,0 +1,54 @@
+package org.pkl.core
+
+import org.assertj.core.api.Assertions.assertThat
+import org.fusesource.jansi.Ansi
+import org.junit.jupiter.api.*
+
+class ErrorColoringTest {
+  companion object {
+    val evaluator by lazy { Evaluator.preconfigured() }
+
+    @AfterAll
+    @JvmStatic
+    fun afterAll() {
+      evaluator.close()
+    }
+  }
+
+  private fun evaluate(program: String, expression: String): Any {
+    return evaluator.evaluateExpression(ModuleSource.text(program), expression)
+  }
+  
+  @BeforeEach
+  fun setup() {
+    // Enable colouring before each test
+    Ansi.setEnabled(true)
+  }
+  
+  @AfterEach
+  fun teardown() {
+    // Disable colouring after each test
+    Ansi.setEnabled(false)
+  }
+  
+  @Test
+  fun `simple error`() {
+    val error = assertThrows<PklException> { evaluate("bar = 2", "bar = 15") }
+
+    assertThat(error).message()
+      .contains("\u001B[31m–– Pkl Error ––\u001B[m")
+      .contains("\u001B[94m1 | \u001B[m")
+      .contains("\u001B[0;31m^")
+  }
+  
+  @Test
+  fun `repeated error`() {
+    val error = assertThrows<PklException> { evaluate("""self: String = "Strings; if they were lazy, you could tie the knot on \(self.take(7))"""", "self") }
+    assertThat(error).message()
+      .contains("A stack overflow occurred.")
+      .contains("┌─ ")
+      .contains(" repetitions of:")
+      .contains("│ 1 | ")
+      .contains("^^^^")
+  }
+}