[gdb/testsuite] Don't abort testrun for invalid command in test-case

Say we add a call to foobar at the end of a test-case, and run the
test-suite.  We'll run into a dejagnu error:
...
ERROR: (DejaGnu) proc "foobar" does not exist.
...
and the test-suite run is aborted.

It's reasonable that the test-case is aborted, but it's not reasonable that
the testsuite run is aborted.

Problems in one test-case should not leak into other test-cases, and they
generally don't.  The exception is the "invalid command name" problem due to
an override of ::unknown in dejagnu's framework.exp.

Fix this by limiting the scope of dejagnu's ::unknown override.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-11  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26110
	* lib/gdb.exp (gdb_unknown): New proc.
	(::tcl_source): Rename from ::source.
	(source): New proc.

---
 gdb/testsuite/lib/gdb.exp | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 51f8a05464..ae6d561a10 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -7243,5 +7243,35 @@ proc with_override { name override body } {
     return $result
 }
 
+proc gdb_unknown { args } {
+    # Skip dejagnu_unknown to prevent it from exiting and aborting the entire
+    # test run.
+    return [uplevel 1 ::tcl_unknown $args]
+}
+
+rename ::source ::tcl_source
+
+proc source { arg } {
+    set script $arg
+
+    set script [file dirname $script]
+    set subdir3 [file tail $script]
+
+    set script [file dirname $script]
+    set subdir2 [file tail $script]
+
+    set script [file dirname $script]
+    set subdir1 [file tail $script]
+
+    if { $subdir1 == "gdb"
+        && $subdir2 == "testsuite"
+        && [regexp {^gdb[.]} $subdir3] } {
+       return \
+           [uplevel 1 with_override ::unknown gdb_unknown \"::tcl_source $arg\"]
+    }
+
+    return [uplevel 1 ::tcl_source $arg]
+}
+
 # Always load compatibility stuff.
 load_lib future.exp