chore(test): Replace try/fail/catch/assert with assertThat

[guicamest]

Description of changes:
This PR replaces several occurrences of the idiom:

try {
  ... // do something that throws an Exception
  fail() // with some message to display because the exception should have been thrown
} catch ( someException ){
  ... // some assertion on the exception
}

with a couple of assertion methods provided by AssertJ (assertThatCode, assertThatThrownBy) and JUnit (assertThrows) which are already part of the dependencies of the project.

Before:

        try {
            provider.newFileSystem(URI.create(pathUri));
            fail("filesystem created twice!");
        } catch (FileSystemAlreadyExistsException x) {
            assertTrue(x.getMessage().contains("'foo'"));
        }

After:

assertThatCode(() -> provider.newFileSystem(URI.create(pathUri)))
                .as("filesystem created twice!")
                .isInstanceOf(FileSystemAlreadyExistsException.class)
                .hasMessageContaining("'foo'");

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[markjschreiber]

Nice refactoring to assertJ idioms. Thanks!