tests: Include tests in coverage metric

l0b0 · l0b0 · commit 3e5fed638e69 · 2021-06-29T15:44:28.000+12:00
Missing coverage in tests means unreached (and usually unreachable) test
code, often indicating a test bug.
diff --git a/.coveragerc b/.coveragerc
@@ -2,4 +2,6 @@
 fail_under = 91
 
 [run]
-source = pystac
+source =
+    pystac
+    tests
