fix(coverage): merge duplicate reports (#8942)

Merging multiple runs isn't quite right because we 
rely on a 0 count to signal that a block hasn't been called.

Other tools like c8 expect this to be true as-well so we 
need to do our best to merge coverage files rather 
than duplicating them.

4 files changed, 48 insertions, 9 deletions

diff --git a/cli/tests/integration_tests.rs b/cli/tests/integration_tests.rs
index 9dc4df7cb..199172e2c 100644
--- a/cli/tests/integration_tests.rs
+++ b/cli/tests/integration_tests.rs
@@ -3330,6 +3330,12 @@ itest!(deno_test_run_run_coverage {
   exit_code: 0,
 });
 
+itest!(deno_test_run_combined_coverage {
+  args: "test --allow-all --coverage --unstable test_run_run_coverage.ts test_run_test_coverage.ts",
+  output: "test_run_combined_coverage.out",
+  exit_code: 0,
+});
+
 itest!(deno_lint {
   args: "lint --unstable lint/file1.js lint/file2.ts lint/ignored_file.ts",
   output: "lint/expected.out",
diff --git a/cli/tests/run_coverage.ts b/cli/tests/run_coverage.ts
index 098d463e8..d1443dfad 100644
--- a/cli/tests/run_coverage.ts
+++ b/cli/tests/run_coverage.ts
@@ -1,3 +1,3 @@
-import { returnsFoo2 } from "./subdir/mod1.ts";
+import { returnsHi } from "./subdir/mod1.ts";
 
-returnsFoo2();
+returnsHi();
diff --git a/cli/tests/test_run_combined_coverage.out b/cli/tests/test_run_combined_coverage.out
new file mode 100644
index 000000000..476e9adcc
--- /dev/null
+++ b/cli/tests/test_run_combined_coverage.out
@@ -0,0 +1,31 @@
+Check [WILDCARD]/tests/$deno$test.ts
+running 2 tests
+test spawn test ... Check [WILDCARD]/tests/run_coverage.ts
+ok ([WILDCARD])
+test spawn test ... Check [WILDCARD]/tests/$deno$test.ts
+running 1 tests
+test returnsFooSuccess ... ok ([WILDCARD])
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out ([WILDCARD])
+
+ok ([WILDCARD])
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out ([WILDCARD])
+
+cover [WILDCARD]/tests/run_coverage.ts ... 100.000% (3/3)
+cover [WILDCARD]/tests/subdir/mod1.ts ... 57.143% (8/14)
+   8 | export function printHello3() {
+   9 |     printHello2();
+  10 | }
+  11 | export function throwsError() {
+  12 |     throw Error("exception from mod1");
+  13 | }
+cover [WILDCARD]/tests/subdir/print_hello.ts ... 25.000% (1/4)
+   1 | export function printHello() {
+   2 |     console.log("Hello");
+   3 | }
+cover [WILDCARD]/tests/subdir/subdir2/mod2.ts ... 62.500% (5/8)
+   5 | export function printHello2() {
+   6 |     printHello();
+   7 | }
+cover [WILDCARD]/tests/test_coverage.ts ... 100.000% (5/5)
diff --git a/cli/tests/test_run_run_coverage.out b/cli/tests/test_run_run_coverage.out
index 15ea230ef..2f18020d3 100644
--- a/cli/tests/test_run_run_coverage.out
+++ b/cli/tests/test_run_run_coverage.out
@@ -1,16 +1,15 @@
-Check [WILDCARD]/$deno$test.ts
+Check [WILDCARD]/tests/$deno$test.ts
 running 1 tests
-test spawn test ... Check [WILDCARD]/run_coverage.ts
+test spawn test ... Check [WILDCARD]/tests/run_coverage.ts
 ok ([WILDCARD])
 
 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out ([WILDCARD])
 
 cover [WILDCARD]/tests/run_coverage.ts ... 100.000% (3/3)
 cover [WILDCARD]/tests/subdir/mod1.ts ... 35.714% (5/14)
-   2 | export function returnsHi() {
-   3 |     return "Hi";
-   4 | }
------|-----
+   5 | export function returnsFoo2() {
+   6 |     return returnsFoo();
+   7 | }
    8 | export function printHello3() {
    9 |     printHello2();
   10 | }
@@ -21,7 +20,10 @@ cover [WILDCARD]/tests/subdir/print_hello.ts ... 25.000% (1/4)
    1 | export function printHello() {
    2 |     console.log("Hello");
    3 | }
-cover [WILDCARD]/tests/subdir/subdir2/mod2.ts ... 62.500% (5/8)
+cover [WILDCARD]/tests/subdir/subdir2/mod2.ts ... 25.000% (2/8)
+   2 | export function returnsFoo() {
+   3 |     return "Foo";
+   4 | }
    5 | export function printHello2() {
    6 |     printHello();
    7 | }