[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.

(cherry picked from commit 6635337)

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")
     }
   }