cli: test + misc improvements (#1083)

* cli: Added --max-examples-per-bucket flag to 'kopia snapshot estimate'

Added and cleaned up a bunch of unit tests.

Fixes #1054

* cli: misc tests to increase code coverage of the cli package

* ci: move code coverage run into separate GH job

.github/workflows/code-coverage.yml

@@ -0,0 +1,20 @@
+name: Code Coverage
+on: [push,pull_request]
+jobs:
+  tests:
+    name: Code Coverage Tests
+    runs-on: ubuntu-latest
+    steps:
+    - name: Set up Go.
+      uses: actions/setup-go@v2
+      with:
+        go-version: ^1.16
+      id: go
+    - name: Check out code into the Go module directory
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - name: Run Tests
+      run: make test-with-coverage
+    - name: Publish Coverage Results
+      run: make ci-publish-coverage