Don't implicitly infer test context. (#1686)

(cherry picked from commit a7e2befc8fe76f88d611a29f3f2e08ea19120444)

# Conflicts:
#	Tests/DependenciesTests/DependencyValuesTests.swift

Author: mbrandonw

Sources/Dependencies/DependencyValues.swift

@@ -298,8 +298,6 @@ struct CurrentDependency {
 private let defaultContext: DependencyContext = {
   if ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] == "1" {
     return .preview
-  } else if _XCTIsTesting {
-    return .test
   } else {
     return .live
   }

Tests/DependenciesTests/DependencyValuesTests.swift

@@ -68,16 +68,19 @@ final class DependencyValuesTests: XCTestCase {
 
   func testDependencyDefaultIsReused() {
     DependencyValues.withValue(\.self, .init()) {
+        DependencyValues.withValue(\.context, .test) {
       @Dependency(\.reuseClient) var reuseClient: ReuseClient
 
       XCTAssertEqual(reuseClient.count(), 0)
       reuseClient.setCount(42)
       XCTAssertEqual(reuseClient.count(), 42)
     }
   }
+  }
 
   func testDependencyDefaultIsReused_SegmentedByContext() {
     DependencyValues.withValue(\.self, .init()) {
+      DependencyValues.withValue(\.context, .test) {
       @Dependency(\.reuseClient) var reuseClient: ReuseClient
 
       XCTAssertEqual(reuseClient.count(), 0)
@@ -116,6 +119,7 @@ final class DependencyValuesTests: XCTestCase {
       XCTAssertEqual(reuseClient.count(), 42)
     }
   }
+  }
 
   func testAccessingTestDependencyFromLiveContext_WhenUpdatingDependencies() {
     @Dependency(\.reuseClient) var reuseClient: ReuseClient
@@ -142,6 +146,7 @@ final class DependencyValuesTests: XCTestCase {
   }
 
   func testBinding() {
+    DependencyValues.withValue(\.context, .test) {
     @Dependency(\.childDependencyEarlyBinding) var childDependencyEarlyBinding:
       ChildDependencyEarlyBinding
     @Dependency(\.childDependencyLateBinding) var childDependencyLateBinding:
@@ -179,6 +184,7 @@ final class DependencyValuesTests: XCTestCase {
       XCTAssertEqual(childDependencyLateBindingEscaped.fetch(), 1_000)
     }
   }
+  }
 
   func testNestedDependencyIsOverridden() {
     DependencyValues.withValue(\.nestedValue.value, 10) {