TermStatusLine: fix python 2.7 and add test (#28715)

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

lib/spack/spack/installer.py

@@ -683,7 +683,7 @@ def clear(self):
             return
 
         self.pkg_set.clear()
-        self.pkg_list.clear()
+        self.pkg_list = []
 
         # Move the cursor to the beginning of the first "Waiting for" message and clear
         # everything after it.

lib/spack/spack/test/installer.py

@@ -1272,3 +1272,14 @@ def remove(self, spec):
     # Make sure that `remove` was called on the database after an unsuccessful
     # attempt to restore the backup.
     assert fake_db.called
+
+
+def test_term_status_line():
+    # Smoke test for TermStatusLine; to actually test output it would be great
+    # to pass a StringIO instance, but we use tty.msg() internally which does not
+    # accept that. `with log_output(buf)` doesn't really work because it trims output
+    # and we actually want to test for escape sequences etc.
+    x = inst.TermStatusLine(enabled=True)
+    x.add("a")
+    x.add("b")
+    x.clear()