test: Measure and track code coverage (#2636)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
- Added coverage badge to README. - [See my
fork](https://github.com/ChristianZaccaria/llama-stack)
- Added a GitHub Actions workflow that runs the tests and updates the
coverage badge. - [See
run](4574811323)
- Documented steps in `testing.md` for running the tests locally, and
viewing the `html` report.
- Excluded non-essential files from coverage reporting to provide a more
accurate measurement.

Automatically created PR to update coverage badge:
https://github.com/ChristianZaccaria/llama-stack/pull/9

# Note for reviewers
1. Currently the coverage report shows a 45% coverage. Wondering if
there are other files or directories that should also be excluded from
the report to increase the percentage. The directories with the least
test coverage are `llama_stack/cli`, `llama_stack/models`, and
`llama_stack/ui`. - Should we exclude these?
2. **[Required]** The `GITHUB_TOKEN` should have write permissions to
open a PR to update the coverage badge.

# GitHub Issue
<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->
Closes #2355 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->
The `testing.md` file describes how to run the unit tests locally.

coverage.svg

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="99" height="20">
+    <linearGradient id="b" x2="0" y2="100%">
+        <stop offset="0" stop-color="#bbb" stop-opacity=".1"/>
+        <stop offset="1" stop-opacity=".1"/>
+    </linearGradient>
+    <mask id="a">
+        <rect width="99" height="20" rx="3" fill="#fff"/>
+    </mask>
+    <g mask="url(#a)">
+        <path fill="#555" d="M0 0h63v20H0z"/>
+        <path fill="#fe7d37" d="M63 0h36v20H63z"/>
+        <path fill="url(#b)" d="M0 0h99v20H0z"/>
+    </g>
+    <g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="11">
+        <text x="31.5" y="15" fill="#010101" fill-opacity=".3">coverage</text>
+        <text x="31.5" y="14">coverage</text>
+        <text x="80" y="15" fill="#010101" fill-opacity=".3">44%</text>
+        <text x="80" y="14">44%</text>
+    </g>
+</svg>