This change removes non-useful messages from assertions in
org.eclipse.core.tests.resources.regression. In cases where multiple
assertions without proper messages are placed in subsequent lines of
code, assertions are improved to provide better messages and help
identifying the failing line of code. This particularly affects
assertTrue/assertFalse statements that are replaced with
assertThat().matches().

This also prepares for a migration of assertions to JUnit 5, which would
otherwise require swapping of parameters, as the message parameter has
been moved to the end of the parameter list.

Contributes to
https://github.com/eclipse-platform/eclipse.platform/issues/903