Fixed trimming of the recorded test output

This fixes a bug in our test framework. It seems like
`Runtime.getRuntime().freeMemory()` does not indicate the total memory that can
still be allocated, but only the free memory that is already allocated by the
runtime. Therefore, the recorded output was often cut prematurely.
This fix only considers the size of the recorded string buffer. It also makes
sure, that the buffer is trimmed after deleting elements.

core/src/testFixtures/java/org/lflang/tests/LFTest.java

@@ -258,11 +258,16 @@ private Thread recordStream(StringBuffer builder, InputStream inputStream) {
               int len;
               char[] buf = new char[1024];
               while ((len = reader.read(buf)) > 0) {
-                if (Runtime.getRuntime().freeMemory() < Runtime.getRuntime().totalMemory() / 2) {
+                builder.append(buf, 0, len);
+                // If the buffer gets too large, then we delete the first half of the buffer
+                // and trim it down in size. It is important to decide what "too large" means.
+                // Here we take 1/4 of the total memory available to the Java runtime as a rule of
+                // thumb.
+                if (builder.length() > Runtime.getRuntime().totalMemory() / 4) {
                   builder.delete(0, builder.length() / 2);
-                  builder.insert(0, "[earlier messages were removed to free up memory]%n");
+                  builder.insert(0, "[earlier messages were removed to free up memory]\n");
+                  builder.trimToSize();
                 }
-                builder.append(buf, 0, len);
               }
             } catch (IOException e) {
               throw new RuntimeIOException(e);