Increase minimum unit test FS image size to 320 MiB (#217)

From 27-Nov-2022 the alpine_test GitLab CI job started failing,
reporting errors creating XFS file systems in the
test_SupportedFileSystems unit test like this:

    [ RUN      ] My/SupportedFileSystemsTest.Create/xfs
    test_SupportedFileSystems.cc:501: Failure
    Value of: m_fs_object->create(m_partition, m_operation_detail)
      Actual: false
    Expected: true
    Operation details:
    mkfs.xfs -f -L '' '/builds/GNOME/gparted/tests/test_SupportedFileSystems.img'    00:00:00  (ERROR)

    Filesystem must be larger than 300MB.
    ...

This is because Docker image "alpine:latest" has updated to Alpine Linux
3.17 which includes xfsprogs 6.0.0 which includes this change (first
released in xfsprogs 5.19.0):

    https://git.kernel.org/pub/scm/fs/xfs/xfsprogs-dev.git/commit/?id=6e0ed3d19c54603f0f7d628ea04b550151d8a262
    mkfs: stop allowing tiny filesystems

    Refuse to format a filesystem that are "too small", because these
    configurations are known to have performance and redundancy problems
    that are not present on the volume sizes that XFS is best at
    handling.

    Specifically, this means that we won't allow logs smaller than 64MB,
    we won't allow single-AG filesystems, and we won't allow volumes
    smaller than 300MB.

Increase the default unit test file system image size from 256 MiB to
256+64 = 320 MiB to avoid this error.

Closes #217 - GitLab CI test job failing with new mkfs.xfs error
              "Filesystem must be larger than 300MB."

tests/test_SupportedFileSystems.cc

@@ -239,7 +239,7 @@ const std::string param_fsname(const ::testing::TestParamInfo<FSType>& info)
 	}
 
 
-const Byte_Value IMAGESIZE_Default = 256*MEBIBYTE;
+const Byte_Value IMAGESIZE_Default = 320*MEBIBYTE;
 const Byte_Value IMAGESIZE_Larger  = 512*MEBIBYTE;