Set placeholder values in the test POM during the test run

[sparsick]

Is your feature request related to a problem? Please describe.
When implementing georgberky/dependency-update-maven-plugin#28 we needed a test where the scm configuration in the test POM <scm> is set during the @BeforeEach phase, where we can define the git repository url.

Describe the solution you'd like
It is already possible to use a placeholder variable in the test POM like

<scm>
  <connectionUrl> ${myPlaceholder}</connectionUrl>
</scm>

The idea is now to set the value of this placeholder variable in the @BeforeEach phase of JUnit5

Describe alternatives you've considered

Other idea could be to manipulate the POM model programmatically in the JUnit5 tests. But IMHO this solution makes it more difficult to the readability of the tests.