Clean up some XCTSkips (#6808)

- enable a test that is passing again
- remove some outdated skipping code in another test

Author: neonichu

Tests/CommandsTests/BuildToolTests.swift

@@ -77,8 +77,6 @@ final class BuildToolTests: CommandsTestCase {
     }
 
     func testImportOfMissedDepWarning() throws {
-        try XCTSkipIf(true, "tested functionality doesn't work due to 'libSwiftScan dependency scan query failed' (rdar://108512158)")
-
         // Verify the warning flow
         try fixture(name: "Miscellaneous/ImportOfMissingDependency") { path in
             let fullPath = try resolveSymlinks(path)

Tests/SourceControlTests/RepositoryManagerTests.swift

@@ -126,14 +126,6 @@ class RepositoryManagerTests: XCTestCase {
     }
 
     func testCache() throws {
-        if #available(macOS 11, *) {
-            // No need to skip the test.
-        }
-        else {
-            // Avoid a crasher that seems to happen only on macOS 10.15, but leave an environment variable for testing.
-            try XCTSkipUnless(ProcessEnv.vars["SWIFTPM_ENABLE_FLAKY_REPOSITORYMANAGERTESTS"] == "1", "skipping test that sometimes crashes in CI (rdar://70540298)")
-        }
-
         let fs = localFileSystem
         let observability = ObservabilitySystem.makeForTesting()