Add guides for test runner (#4308)

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+---
+name: Set a code coverage threshold with the Bun test runner
+---
+
+Bun's test runner supports built-in code coverage reporting via the `--coverage` flag.
+
+```sh
+$ bun test --coverage
+
+test.test.ts:
+✓ math > add [0.71ms]
+✓ math > multiply [0.03ms]
+✓ random [0.13ms]
+-------------|---------|---------|-------------------
+File         | % Funcs | % Lines | Uncovered Line #s
+-------------|---------|---------|-------------------
+All files    |   66.67 |   77.78 |
+ math.ts     |   50.00 |   66.67 |
+ random.ts   |   50.00 |   66.67 |
+-------------|---------|---------|-------------------
+
+ 3 pass
+ 0 fail
+ 3 expect() calls
+```
+
+---
+
+To set a minimum coverage threshold, add the following line to your `bunfig.toml`. This requires that 90% of your codebase is covered by tests.
+
+```toml
+[test]
+# to require 90% line-level and function-level coverage
+coverageThreshold = 0.9
+```
+
+---
+
+If your test suite does not meet this threshold, `bun test` will exit with a non-zero exit code to signal a failure.
+
+```sh
+$ bun test --coverage
+<test output>
+$ echo $?
+1 # this is the exit code of the previous command
+```
+
+Different thresholds can be set for line-level and function-level coverage.
+
+```toml
+[test]
+# to set different thresholds for lines and functions
+coverageThreshold = { line = 0.5, function = 0.7 }
+```
+
+---
+
+See [Docs > Test runner > Coverage](/docs/test/coverage) for complete documentation on code coverage reporting in Bun.