Run readonly tests with mount dir option

[Tulsishah]

This will be pull request template

Description

Currently, Readonly integration tests are running only for the testbucket flag. I made changes so that for read-only tests, when we pass the mountedDiectory flag, we need to pass the testBucket flag with the mounted bucket, and it will run the tests with the mountedDiectory flag.

Link to the issue in case of a bug fix.

Testing details

1. Manual -

   1. From gcsfuse directory with testBucket flag
      GODEBUG=asyncpreemptoff=1 go test ./tools/integration_tests/... -p 1 --integrationTest -v --testbucket=myBucket
      Results
      All the tests are passing

   2. From cd tool/integration_tests/readonly
      GODEBUG=asyncpreemptoff=1 go test . -p 1 --integrationTest -v --mountedDirectory=/usr/local/google/home/tulsishah/gcs
      Results
      2023/05/04 09:17:24 Please pass testBucket mounted in mountedDirectory.
      FAIL github.com/googlecloudplatform/gcsfuse/tools/integration_tests/readonly 0.006s
      FAIL

   3. From cd tool/integration_tests/readonly
      GODEBUG=asyncpreemptoff=1 go test . -p 1 --integrationTest -v --mountedDirectory=/usr/local/google/home/tulsishah/gcs --testbucket=tulsishah_test
      mountedDirectory mounted for tulsishah_test with --o=ro flag
      Results
      All the tests are working fine

   4. From cd tool/integration_tests/implicitdir
      GODEBUG=asyncpreemptoff=1 go test . -p 1 --integrationTest -v --mountedDirectory=/usr/local/google/home/tulsishah/gcs
      Results
      All the tests are working fine

   5. From cd tool/integration_tests/implicitdir
      GODEBUG=asyncpreemptoff=1 go test . -p 1 --integrationTest -v --mountedDirectory=/usr/local/google/home/tulsishah/gcs --testbucket=tulsishah_test
      Results
      2023/05/04 09:18:15 Both --testbucket and --mountedDirectory can't be specified at the same time.
      FAIL github.com/googlecloudplatform/gcsfuse/tools/integration_tests/implicitdir 0.007s
      FAIL

2. Unit tests - N/A

3. Integration tests - All the integration tests are working fine.