Ignore .coverage* files

When a coverage command aborts, some temporary files related to the
coverage remain like this:
    .coverage.localhost.24409.990268
    .coverage.localhost.24415.503858
    .coverage.localhost.24417.953422
    .coverage.localhost.24419.169526
    .coverage.localhost.24421.143443
    .coverage.localhost.24423.259249
    .coverage.localhost.24425.429360
    .coverage.localhost.24427.025521
    .coverage.localhost.24432.204681
    .coverage.localhost.24479.725555
We should not manage these files in the git repository.

Partially implements bp unit-tests

Change-Id: Ifa5aaa526b27473c5e5220a5be4a910603676e60

.gitignore

@@ -15,5 +15,5 @@ include/swift_objects/swift_large
 dist
 build
 .testrepository
-.coverage
+.coverage*
 cover/