[SPARK-10247] [core] improve readability of a test case in DAGSchedulerSuite

[squito]

This is pretty minor, just trying to improve the readability of DAGSchedulerSuite, I figure every bit helps. Before whenever I read this test, I never knew what "should work" and "should be ignored" really meant -- this adds some asserts & updates comments to make it more clear. Also some reformatting per a suggestion from @markhamstra on #7699

[rxin]

Can we add some blank line to separate different checks?

[SparkQA]

Test build #41561 timed out for PR 8434 at commit 8e2a969 after a configured wait of 175m.

[andrewor14]

retest this please

[andrewor14]

@squito can you add the suggested blank lines? Otherwise this LGTM.

[SparkQA]

Test build #41890 has finished for PR 8434 at commit 8e2a969.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[squito]

@andrewor14 added blank lines (and also expanded comments a teeny bit)

[SparkQA]

Test build #41935 has finished for PR 8434 at commit 94927c8.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[andrewor14]

retest this please

[SparkQA]

Test build #41942 has finished for PR 8434 at commit 94927c8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[andrewor14]

Merged into master, thanks.