[Tests] Remove a check for compiler side diagnostics

Remove a check for the exact diagnostics message from the compiler side as it is out of scope for the test and it prohibits improvements to the diagnostics message from the compiler.

Author: cachemeifyoucan

Tests/SwiftDriverTests/CachingBuildTests.swift

@@ -873,7 +873,6 @@ final class CachingBuildTests: XCTestCase {
 
  XCTAssertEqual(testDiagnostics.count, 1)
  XCTAssertEqual(testDiagnostics[0].severity, .error)
- XCTAssertEqual(testDiagnostics[0].message, "CAS error encountered: conflicting CAS options used in scanning service")
  }
  }