Add integration test for testing SwiftPM's code coverage support

<rdar://problem/45575408> Add integration test for testing SwiftPM's code coverage support

Author: aciidgh

test-codecov-package/foo/.gitignore

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+.DS_Store
+/.build
+/Packages
+/*.xcodeproj

test-codecov-package/foo/Package.swift

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+// swift-tools-version:4.2
+
+import PackageDescription
+
+let package = Package(
+    name: "foo",
+    products: [
+        .library(
+            name: "foo",
+            targets: ["foo"]),
+    ],
+    targets: [
+        .target(
+            name: "foo",
+            dependencies: []),
+        .testTarget(
+            name: "fooTests",
+            dependencies: ["foo"]),
+    ]
+)

test-codecov-package/foo/README.md

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+# foo
+
+A description of this package.

test-codecov-package/foo/Sources/foo/foo.swift

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+public struct foo {
+    public var text = "Hello, World!"
+
+    public init() { }
+}

test-codecov-package/foo/Tests/LinuxMain.swift

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+import XCTest
+
+import fooTests
+
+var tests = [XCTestCaseEntry]()
+tests += fooTests.allTests()
+XCTMain(tests)

test-codecov-package/foo/Tests/fooTests/XCTestManifests.swift

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+import XCTest
+
+#if !os(macOS)
+public func allTests() -> [XCTestCaseEntry] {
+    return [
+        testCase(fooTests.allTests),
+    ]
+}
+#endif

test-codecov-package/foo/Tests/fooTests/fooTests.swift

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+import XCTest
+import foo
+
+final class fooTests: XCTestCase {
+    func testExample() {
+        // This is an example of a functional test case.
+        // Use XCTAssert and related functions to verify your tests produce the correct
+        // results.
+        XCTAssertEqual(foo().text, "Hello, World!")
+    }
+
+    static var allTests = [
+        ("testExample", testExample),
+    ]
+}

test-codecov-package/test-codecov-package.txt

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+// Sanity check for SwiftPM's code coverage support.
+//
+// Make a sandbox dir.
+// RUN: rm -rf %t.dir
+// RUN: mkdir -p %t.dir
+// RUN: cp -r %S/foo %t.dir/
+// RUN: %{swift-test} --package-path %t.dir/foo --enable-code-coverage -v 2>&1 | tee %t.build-log
+//
+// Check the build log.
+//
+// RUN: %{FileCheck} --check-prefix CHECK-BUILD-LOG --input-file %t.build-log %s
+//
+// CHECK-BUILD-LOG: 'fooTests' passed
+//
+// Check the exported codecov file.
+//
+// RUN: %{FileCheck} --check-prefix CHECK-CODECOV-JSON --input-file %t.dir/foo/.build/debug/codecov/foo.json %s
+//
+// CHECK-CODECOV-JSON: llvm.coverage.json.export