feat(test): support DENO_COVERAGE_DIR env var (#28291)

This PR adds support of `DENO_COVERAGE_DIR` for controlling coverage
output.

tests/specs/coverage/set_coverage_in_env_var/__test__.jsonc

@@ -0,0 +1,18 @@
+{
+  "tempDir": true,
+  "steps": [
+    {
+      "args": "test -A",
+      "output": "test.out",
+      "envs": {
+        "DENO_COVERAGE_DIR": "my_coverage_dir"
+      },
+      "exitCode": 0
+    },
+    {
+      "args": "coverage my_coverage_dir",
+      "output": "coverage.out",
+      "exitCode": 0
+    }
+  ]
+}

tests/specs/coverage/set_coverage_in_env_var/coverage.out

@@ -0,0 +1,7 @@
+--------------------------------
+File       | Branch % | Line % |
+--------------------------------
+ source.ts |    100.0 |  100.0 |
+--------------------------------
+ All files |    100.0 |  100.0 |
+--------------------------------

tests/specs/coverage/set_coverage_in_env_var/source.ts

@@ -0,0 +1,3 @@
+export function add(a: number, b: number): number {
+  return a + b;
+}

tests/specs/coverage/set_coverage_in_env_var/test.out

@@ -0,0 +1,7 @@
+Check [WILDCARD]/test.ts
+running 1 test from ./test.ts
+add() ... ok ([WILDCARD])
+
+ok | 1 passed | 0 failed ([WILDCARD])
+
+[WILDCARD]

tests/specs/coverage/set_coverage_in_env_var/test.ts

@@ -0,0 +1,7 @@
+import { add } from "./source.ts";
+
+Deno.test("add()", () => {
+  if (add(1, 2) !== 3) {
+    throw new Error("test failed");
+  }
+});