Fixing an intermittent test failure

[hiranya911]

Not sure how, but the recent changes to database tests is causing a test failure in some environments:

Running com.google.firebase.database.FirebaseDatabaseTest
Tests run: 10, Failures: 0, Errors: 2, Skipped: 0, Time elapsed: 0.052 sec <<< FAILURE! - in com.google.firebase.database.FirebaseDatabaseTest
testDbUrlIsEmulatorUrlForDbRefWithPath(com.google.firebase.database.FirebaseDatabaseTest)  Time elapsed: 0.004 sec  <<< ERROR!
java.lang.IllegalArgumentException: Failed to load settings from the system's environment variables
	at com.google.firebase.database.FirebaseDatabaseTest.testDbUrlIsEmulatorUrlForDbRefWithPath(FirebaseDatabaseTest.java:252)
Caused by: com.fasterxml.jackson.core.JsonParseException: 
Unexpected character (',' (code 44)): was expecting double-quote to start field name
 at [Source: (String)"{,,"; line: 1, column: 3]
	at com.google.firebase.database.FirebaseDatabaseTest.testDbUrlIsEmulatorUrlForDbRefWithPath(FirebaseDatabaseTest.java:252)

testDbUrlIsEmulatorUrlWhenSettingOptionsManually(com.google.firebase.database.FirebaseDatabaseTest)  Time elapsed: 0 sec  <<< ERROR!
java.lang.IllegalArgumentException: Failed to load settings from the system's environment variables
	at com.google.firebase.database.FirebaseDatabaseTest.testDbUrlIsEmulatorUrlWhenSettingOptionsManually(FirebaseDatabaseTest.java:215)
Caused by: com.fasterxml.jackson.core.JsonParseException: 
Unexpected character (',' (code 44)): was expecting double-quote to start field name
 at [Source: (String)"{,,"; line: 1, column: 3]
	at com.google.firebase.database.FirebaseDatabaseTest.testDbUrlIsEmulatorUrlWhenSettingOptionsManually(FirebaseDatabaseTest.java:215)

Seems to be related to the order of tests. FirebaseAppTest has some test cases where they set the FIREBASE_CONFIG env variable to malformed values like {,,. These values are not cleared and are picked up by later tests.