REPLWrapTestCase.test_bash_env(): export variable PS1

The test test_bash_env() relies on PS1 being an environment variable.
Therefore, `export PS1` making it always an environment variable.
Before this, the test relied on the users bashrc to do this.

The variable PS1 is (by default) not an _environment_ variable and thus
not shown by `env`. Any variable can be made into an environment
variable by exporting it. Some people do this for PS1 in their bashrc.

Closes: #631