(testsuite) Prepend relative files with $::srcdir

in an attempt to be able to run the tests successfully from within
autotools' `make distcheck'.

test/lib/completion.exp

@@ -1,4 +1,4 @@
-source ${srcdir}/lib/library.exp
+source $::srcdir/lib/library.exp
 
 
 proc completion_exit {} {

test/lib/library.exp

@@ -858,8 +858,8 @@ proc start_bash {} {
     set TESTDIR [pwd]
     # If `--tool_exec' option not specified, use "bash"
     if {! [info exists TOOL_EXECUTABLE]} {set TOOL_EXECUTABLE bash}
-    exp_spawn $TOOL_EXECUTABLE --rcfile config/bashrc
-    assert_bash_exec {} "$TOOL_EXECUTABLE --rcfile config/bashrc"
+    exp_spawn $TOOL_EXECUTABLE --rcfile $::srcdir/config/bashrc
+    assert_bash_exec {} "$TOOL_EXECUTABLE --rcfile $::srcdir/config/bashrc"
     # Bash < 3.2.41 has a bug where 'history' disappears from SHELLOPTS
     # whenever a shopt setting is sourced or eval'ed.  Disabling 'history'
     # makes it not show in tests "Environment should not be modified"

test/lib/unit.exp

@@ -1,4 +1,4 @@
-source lib/library.exp
+source $::srcdir/lib/library.exp
 
 
 proc unit_exit {} {